In this part of the C# tutorial, we cover operators.

Expressions are constructed from operands and operators. The operators of an expression indicate which operations to apply to the operands. The order of evaluation of operators in an expression is determined by the precedence and associativity of the operators

An operator is a special symbol which indicates a certain process is carried out. Operators in programming languages are taken from mathematics. Programmers work with data. The operators are used to process data. An operand is one of the inputs (arguments) of an operator.

C# operators list

The following table shows a set of operators used in the C# language.

An operator usually has one or two operands. Those operators that work with only one operand are called unary operators. Those who work with two operands are called binary operators. There is also one ternary operator ?:, which works with three operands.

Certain operators may be used in different contexts. For example the + operator. From the above table we can see that it is used in different cases. It adds numbers, concatenates strings or delegates; indicates the sign of a number. We say that the operator is overloaded.

C# unary operators

C# unary operators include: +, -, ++, --, cast operator (), and negation !.

C# sign operators

There are two sign operators: + and -. They are used to indicate or change the sign of a value.

using System;

Console.WriteLine(2);
Console.WriteLine(+2);
Console.WriteLine(-2);

The + and - signs indicate the sign of a value. The plus sign can be used to indicate that we have a positive number. It can be omitted and it is mostly done so.

using System;

int a = 1;
Console.WriteLine(-a);
Console.WriteLine(-(-a));

The minus sign changes the sign of a value.

$ dotnet run
-1
1

This is the output.

C# increment and decrement operators

Incrementing or decrementing a value by one is a common task in programming. C# has two convenient operators for this: ++ and --.

x++;
x = x + 1;
...
y--;
y = y - 1;

The above two pairs of expressions do the same.

using System;

int x = 6;

x++;
x++;

Console.WriteLine(x);

x--;
Console.WriteLine(x);

In the above example, we demonstrate the usage of both operators.

int x = 6;

x++;
x++;

We initiate the x variable to 6. Then we increment x two times. Now the variable equals to 8.

x--;

We use the decrement operator. Now the variable equals to 7.

$ dotnet run
8
7

C# explicit cast operator

The explicit cast operator () can be used to cast a type to another type. Note that this operator works only on certain types.

using System;

float val = 3.2f;
int num = (int) val;

Console.WriteLine(num);

In the example, we explicitly cast a float type to int.

Negation operator

The negation operator (!) reverses the meaning of its operand.

using System;

var isValid = false;

if (!isValid)
{
    Console.WriteLine("The option is not valid");
}

In the example, we build a negative condition: it is executed if the inverse of the expression is valid.

C# assignment operator

The assignment operator = assigns a value to a variable. A variable is a placeholder for a value. In mathematics, the = operator has a different meaning. In an equation, the = operator is an equality operator. The left side of the equation is equal to the right one.

int x = 1;

Here we assign a number to the x variable.

x = x + 1;

The previous expression does not make sense in mathematics. But it is legal in programming. The expression adds 1 to the x variable. The right side is equal to 2 and 2 is assigned to x.

3 = x;

This code example results in syntax error. We cannot assign a value to a literal.

C# concatenating strings

The + operator is also used to concatenate strings.

using System;

Console.WriteLine("Return " + "of " + "the king.");

We join three strings together using string concatenation operator.

$ dotnet run
Return of the king.

C# arithmetic operators

The following is a table of arithmetic operators in C#.

The following example shows arithmetic operations.

using System;

int a = 10;
int b = 11;
int c = 12;

int add = a + b + c;
int sb = c - a;
int mult = a * b;
int div = c / 3;
int rem = c % a;

Console.WriteLine(add);
Console.WriteLine(sb);
Console.WriteLine(mult);
Console.WriteLine(div);
Console.WriteLine(rem);

In the preceding example, we use addition, subtraction, multiplication, division, and remainder operations. This is all familiar from the mathematics.

int rem = c % a;

The % operator is called the remainder or the modulo operator. It finds the remainder of division of one number by another. For example, 9 % 4, 9 modulo 4 is 1, because 4 goes into 9 twice with a remainder of 1.

$ dotnet run
33
2
110
4
2

Next we show the distinction between integer and floating point division.

using System;

int c = 5 / 2;
Console.WriteLine(c);

double d = 5 / 2.0;
Console.WriteLine(d);

In the preceding example, we divide two numbers.

int c = 5 / 2;
Console.WriteLine(c);

In this code, we have done integer division. The returned value of the division operation is an integer. When we divide two integers the result is an integer.

double d = 5 / 2.0;
Console.WriteLine(d);

If one of the values is a double or a float, we perform a floating point division. In our case, the second operand is a double so the result is a double.

$ dotnet run
2
2.5

C# Boolean operators

In C#, we have three logical operators. The bool keyword is used to declare a Boolean value.

Boolean operators are also called logical.

using System;

int x = 3;
int y = 8;

Console.WriteLine(x == y);
Console.WriteLine(y > x);

if (y > x)
{
    Console.WriteLine("y is greater than x");
}

Many expressions result in a boolean value. Boolean values are used in conditional statements.

Console.WriteLine(x == y);
Console.WriteLine(y > x);

Relational operators always result in a boolean value. These two lines print false and true.

if (y > x)
{
    Console.WriteLine("y is greater than x");
}

The body of the if statement is executed only if the condition inside the parentheses is met. The y > x returns true, so the message "y is greater than x" is printed to the terminal.

The true and false keywords represent boolean literals in C#.

using System;

bool a = true && true;
bool b = true && false;
bool c = false && true;
bool d = false && false;

Console.WriteLine(a);
Console.WriteLine(b);
Console.WriteLine(c);
Console.WriteLine(d);

Example shows the logical and operator. It evaluates to true only if both operands are true.

$ dotnet run
True
False
False
False

Only one expression results in True.

The logical or || operator evaluates to true, if either of the operands is true.

using System;

bool a = true || true;
bool b = true || false;
bool c = false || true;
bool d = false || false;

Console.WriteLine(a);
Console.WriteLine(b);
Console.WriteLine(c);
Console.WriteLine(d);

If one of the sides of the operator is true, the outcome of the operation is true.

$ dotnet run
True
True
True
False

Three of four expressions result in true.

The negation operator ! makes true false and false true.

using System;

Console.WriteLine(!true);
Console.WriteLine(!false);
Console.WriteLine(!(4 < 3));

The example shows the negation operator in action.

$ dotnet run
False
True
True

The ||, and && operators are short circuit evaluated. Short circuit evaluation means that the second argument is only evaluated if the first argument does not suffice to determine the value of the expression: when the first argument of the logical and evaluates to false, the overall value must be false; and when the first argument of logical or evaluates to true, the overall value must be true. Short circuit evaluation is used mainly to improve performance.

An example may clarify this a bit more.

using System;

Console.WriteLine("Short circuit");

if (One() && Two())
{
    Console.WriteLine("Pass");
}

Console.WriteLine("#############");

if (Two() || One())
{
    Console.WriteLine("Pass");
}

bool One()
{
    Console.WriteLine("Inside one");

    return false;
}

bool Two()
{
    Console.WriteLine("Inside two");

    return true;
}

We have two methods in the example. They are used as operands in boolean expressions.

if (One() && Two())
{
    Console.WriteLine("Pass");
}

The One method returns false. The short circuit && does not evaluate the second method. It is not necessary. Once an operand is false, the result of the logical conclusion is always false. Only "Inside one" is only printed to the console.

Console.WriteLine("#############");

if (Two() || One())
{
    Console.WriteLine("Pass");
}

In the second case, we use the || operator and use the Two method as the first operand. In this case, "Inside two" and "Pass" strings are printed to the terminal. It is again not necessary to evaluate the second operand, since once the first operand evaluates to true, the logical or is always true.

$ dotnet run
Short circuit
Inside one
#############
Inside two
Pass

C# relational operators

Relational operators are used to compare values. These operators always result in boolean value.

Relational operators are also called comparison operators.

using System;

Console.WriteLine(3 < 4);
Console.WriteLine(3 == 4);
Console.WriteLine(4 >= 3);
Console.WriteLine(4 != 3);

In the code example, we have four expressions. These expressions compare integer values. The result of each of the expressions is either true or false. In C# we use == to compare numbers. Some languages like Ada, Visual Basic, or Pascal use = for comparing numbers.

C# bitwise operators

Decimal numbers are natural to humans. Binary numbers are native to computers. Binary, octal, decimal, or hexadecimal symbols are only notations of the same number. Bitwise operators work with bits of a binary number. Bitwise operators are seldom used in higher level languages like C#.

The bitwise negation operator changes each 1 to 0 and 0 to 1.

Console.WriteLine(~ 7); // prints -8
Console.WriteLine(~ -8); // prints 7

The operator reverts all bits of a number 7. One of the bits also determines, whether the number is negative or not. If we negate all the bits one more time, we get number 7 again.

The bitwise and operator performs bit-by-bit comparison between two numbers. The result for a bit position is 1 only if both corresponding bits in the operands are 1.

      00110
   &  00011
   =  00010

The first number is a binary notation of 6, the second is 3, and the result is 2.

Console.WriteLine(6 & 3); // prints 2
Console.WriteLine(3 & 6); // prints 2

The bitwise or operator performs bit-by-bit comparison between two numbers. The result for a bit position is 1 if either of the corresponding bits in the operands is 1.

     00110
   | 00011
   = 00111

The result is 00110 or decimal 7.

Console.WriteLine(6 | 3); // prints 7
Console.WriteLine(3 | 6); // prints 7

The bitwise exclusive or operator performs bit-by-bit comparison between two numbers. The result for a bit position is 1 if one or the other (but not both) of the corresponding bits in the operands is 1.

      00110
   ^  00011
   =  00101

The result is 00101 or decimal 5.

Console.WriteLine(6 ^ 3); // prints 5
Console.WriteLine(3 ^ 6); // prints 5

C# compound assignment operators

The compound assignment operators consist of two operators. They are shorthand operators.

a = a + 3;
a += 3;

The += compound operator is one of these shorthand operators. The above two expressions are equal. Value 3 is added to the a variable.

Other compound operators are:

-=   *=   /=   %=   &=   |=   <<=   >>=

using System;

int a = 1;
a = a + 1;

Console.WriteLine(a);

a += 5;
Console.WriteLine(a);

a *= 3;
Console.WriteLine(a);

In the example, we use two compound operators.

int a = 1;
a = a + 1;

The a variable is initiated to one. 1 is added to the variable using the non-shorthand notation.

a += 5;

Using a += compound operator, we add 5 to the a variable. The statement is equal to a = a + 5;.

a *= 3;

Using the *= operator, the a is multiplied by 3. The statement is equal to a = a * 3;.

$ dotnet run
2
7
21

C# new operator

The new operator is used to create objects and invoke constructors.

using System;

var b = new Being();
Console.WriteLine(b);

var vals = new int[] { 1, 2, 3, 4, 5 };
Console.WriteLine(string.Join(" ", vals));

class Being
{
    public Being()
    {
        Console.WriteLine("Being created");
    }
}

In the example, we create a new custom object and a array of integers utilizing the new operator.

public Being()
{
    Console.WriteLine("Being created");
}

This is a constructor. It is called at the time of the object creation.

$ dotnet run
Being created
Being
1 2 3 4 5

C# access operator

The access operator [] is used with arrays, indexers, and attributes.

using System;
using System.Collections.Generic;

var vals = new int[] { 2, 4, 6, 8, 10 };
Console.WriteLine(vals[0]);

var domains = new Dictionary()
{
    { "de", "Germany" },
    { "sk", "Slovakia" },
    { "ru", "Russia" }
};

Console.WriteLine(domains["de"]);

oldMethod();

[Obsolete("Don't use OldMethod, use NewMethod instead", false)]
void oldMethod()
{
    Console.WriteLine("oldMethod()");
}

void newMethod()
{
    Console.WriteLine("newMethod()");
}

In the example, we use the [] operator to get an element of an array, value of a dictionary pair, and activate a built-in attribute.

var vals = new int[] { 2, 4, 6, 8, 10 };
Console.WriteLine(vals[0]);

We define an array of integers. We get the first element with vals[0].

var domains = new Dictionary<string, string>()
{
    { "de", "Germany" },
    { "sk", "Slovakia" },
    { "ru", "Russia" }
};

Console.WriteLine(domains["de"]);

A dictionary is created. With domains["de"], we get the value of the pair that has the "de" key.

[Obsolete("Don't use OldMethod, use NewMethod instead", false)]
public static void oldMethod()
{
    Console.WriteLine("oldMethod()");
}

We active the built-in Obsolete attribute. The attribute issues a warning.

When we run the program, it produces the warning: warning CS0618: 'oldMethod()' is obsolete: 'Don't use OldMethod, use NewMethod instead'.

C# index from end operator ^

The index from end operator ^ indicates the element position from the end of a sequence. For instance, ^1 points to the last element of a sequence and ^n points to the element with offset length - n.

using System;
using System.Linq;

int[] vals = { 1, 2, 3, 4, 5 };

Console.WriteLine(vals[^1]);
Console.WriteLine(vals[^2]);

var word = "gray falcon";

Console.WriteLine(word[^1]);

In the example, we apply the operator on an array and a string.

int[] vals = { 1, 2, 3, 4, 5 };

Console.WriteLine(vals[^1]);
Console.WriteLine(vals[^2]);

We print the last and the last but one element of the array.

var word = "gray falcon";

Console.WriteLine(word[^1]);

We print the last letter of the word.

$ dotnet run
5
4
n

C# range operator ..

The .. operator specifies the start and end of a range of indices as its operands. The left-hand operand is an inclusive start of a range. The right-hand operand is an exclusive end of a range.

x.. is equivalent to x..^0
..y is equivalent to 0..y
.. is equivalent to 0..^0

Operands of the .. operator can be omitted to get an open-ended range.

using System;

int[] vals = { 1, 2, 3, 4, 5, 6, 7 };

var slice1 = vals[1..4];
Console.WriteLine("[{0}]", string.Join(", ", slice1));

var slice2 = vals[..^0];
Console.WriteLine("[{0}]", string.Join(", ", slice2));

In the example, we use the .. operator to get array slices.

var range1 = vals[1..4];
Console.WriteLine("[{0}]", string.Join(", ", range1));

We create an array slice from index 1 till index 4; the last index 4 is not included.

var slice2 = vals[..^0];
Console.WriteLine("[{0}]", string.Join(", ", slice2));

Here we esentially create a copy of the array.

$ dotnet run
[2, 3, 4]
[1, 2, 3, 4, 5, 6, 7]

C# type information

Now we will concern ourselves with operators that work with types.

The sizeof operator is used to obtain the size in bytes for a value type. The typeof is used to obtain the System.Type object for a type.

using System;

Console.WriteLine(sizeof(int));
Console.WriteLine(sizeof(float));
Console.WriteLine(sizeof(Int32));

Console.WriteLine(typeof(int));
Console.WriteLine(typeof(float));

We use the sizeof and typeof operators.

$ dotnet run
4
4
4
System.Int32

We can see that the int type is an alias for System.Int32 and the float is an alias for the System.Single type.

The is operator checks if an object is compatible with a given type.

using System;

Base _base = new Base();
Derived derived = new Derived();

Console.WriteLine(_base is Base);
Console.WriteLine(_base is Object);
Console.WriteLine(derived is Base);
Console.WriteLine(_base is Derived);

class Base { }
class Derived : Base { }

We create two objects from user defined types.

class Base {}
class Derived : Base {}

We have a Base and a Derived class. The Derived class inherits from the Base class.

Console.WriteLine(_base is Base);
Console.WriteLine(_base is Object);

Base equals Base and so the first line prints True. The Base is also compatible with Object type. This is because each class inherits from the mother of all classes — the Object class.

Console.WriteLine(derived is Base);
Console.WriteLine(_base is Derived);

The derived object is compatible with the Base class because it explicitly inherits from the Base class. On the other hand, the _base object has nothing to do with the Derived class.

$ dotnet run
True
True
True
False

The as operator is used to perform conversions between compatible reference types. When the conversion is not possible, the operator returns null. Unlike the cast operation which raises an exception.

using System;

object[] objects = new object[6];
objects[0] = new Base();
objects[1] = new Derived();
objects[2] = "ZetCode";
objects[3] = 12;
objects[4] = 1.4;
objects[5] = null;

for (int i = 0; i < objects.Length; i++)
{
    string s = objects[i] as string;
    Console.Write("{0}:", i);

    if (s != null)
    {
        Console.WriteLine(s);
    }
    else
    {
        Console.WriteLine("not a string");
    }
}

class Base { }
class Derived : Base { }

In the above example, we use the as operator to perform casting.

string s = objects[i] as string;

We try to cast various types to the string type. But only once the casting is valid.

$ dotnet run
0:not a string
1:not a string
2:ZetCode
3:not a string
4:not a string
5:not a string

C# operator precedence

The operator precedence tells us which operators are evaluated first. The precedence level is necessary to avoid ambiguity in expressions.

What is the outcome of the following expression, 28 or 40?

3 + 5 * 5

Like in mathematics, the multiplication operator has a higher precedence than addition operator. So the outcome is 28.

(3 + 5) * 5

To change the order of evaluation, we can use parentheses. Expressions inside parentheses are always evaluated first.

The following table shows common C# operators ordered by precedence (highest precedence first):

Operators on the same row of the table have the same precedence.

using System;

Console.WriteLine(3 + 5 * 5);
Console.WriteLine((3 + 5) * 5);

Console.WriteLine(! true | true);
Console.WriteLine(! (true | true));

In this code example, we show a few expressions. The outcome of each expression is dependent on the precedence level.

Console.WriteLine(3 + 5 * 5);

This line prints 28. The multiplication operator has a higher precedence than addition. First, the product of 5*5 is calculated, then 3 is added.

Console.WriteLine(! true | true);

In this case, the negation operator has a higher precedence. First, the first true value is negated to false, then the | operator combines false and true, which gives true in the end.

$ dotnet run
28
40
True
False

C# associativity rule

Sometimes the precedence is not satisfactory to determine the outcome of an expression. There is another rule called associativity. The associativity of operators determines the order of evaluation of operators with the same precedence level.

9 / 3 * 3

What is the outcome of this expression, 9 or 1? The multiplication, deletion and the modulo operator are left to right associated. So the expression is evaluated this way: (9 / 3) * 3 and the result is 9.

Arithmetic, boolean, relational, and bitwise operators are all left to right associated.

On the other hand, the assignment operator is right associated.

using System;

int a, b, c, d;
a = b = c = d = 0;

Console.WriteLine("{0} {1} {2} {3}", a, b, c, d);

int j = 0;
j *= 3 + 1;

Console.WriteLine(j);

In the example, we have two cases where the associativity rule determines the expression.

int a, b, c, d;
a = b = c = d = 0;

The assignment operator is right to left associated. If the associativity was left to right, the previous expression would not be possible.

int j = 0;
j *= 3 + 1;

The compound assignment operators are right to left associated. We might expect the result to be 1. But the actual result is 0. Because of the associativity. The expression on the right is evaluated first and than the compound assignment operator is applied.

$ dotnet run
0 0 0 0
0

C# null-conditional operator

A null-conditional operator applies a member access, ?., or element access, ?[], operation to its operand only if that operand evaluates to non-null. If the operand evaluates to null, the result of applying the operator is null.

using System;
using System.Collections.Generic;

var users = new List<User>() { new User("John Doe", "gardener"), new User(),
                new User("Lucia Newton", "teacher") };

users.ForEach(user => Console.WriteLine(user.name?.ToUpper()));

class User
{
    public User() { }

    public User(string name, string occupation)
    {
        this.name = name;
        this.occupation = occupation;
    }

    public string name { get; set; }
    public string occupation { get; set; }

    public override string ToString() => $"{name} {occupation}";
}

In the example, we have a User class with two members: name and occupation. We access the name member of the objects with the help of the ?. operator.

var users = new List<User>() { new User("John Doe", "gardener"), new User(),
    new User("Lucia Newton", "teacher") };

We have a list of users. One of them is not initialized and so its members are null.

users.ForEach(user => Console.WriteLine(user.name?.ToUpper()));

We use the ?. to access the name member and call the ToUpper method. The ?. prevents the System.NullReferenceException by not calling the ToUpper on the null value.

$ dotnet run
JOHN DOE

LUCIA NEWTON

In the following example, we use the ?[] operator. The operator allows to place null values into a collection.

using System;

int?[] vals = { 1, 2, 3, null, 4, 5 };

int i = 0;

while (i < vals.Length)
{
    Console.WriteLine(vals[i]?.GetType());
    i++;
}

In this example, we have a null value in an array. We prevent the System.NullReferenceException by applying the ?. operator on the array elements.

C# null-coalescing operator

The null-coalescing operator ?? is used to define a default value for a nullable type. It returns the left-hand operand if it is not null; otherwise it returns the right operand. When we work with databases, we often deal with absent values. These values come as nulls to the program. This operator is a convenient way to deal with such situations.

using System;

int? x = null;
int? y = null;

int z = x ?? y ?? -1;

Console.WriteLine(z);

An example program for null-coalescing operator.

int? x = null;
int? y = null;

Two nullable int types are initiated to null. The int? is a shorthand for Nullable<int>. It allows to have null values assigned to int types.

int z = x ?? y ?? -1;

We want to assign a value to z variable. But it must not be null. This is our requirement. We can easily use the null-coalescing operator for that. In case both x and y variables are null, we assign -1 to z.

$ dotnet run
-1

C# null-coalescing assignment operator

The null-coalescing assignment operator ??= assigns the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null. The ??= operator does not evaluate its right-hand operand if the left-hand operand evaluates to non-null. It is available in C# 8.0 and later.

using System;
using System.Collections.Generic;

List<int> vals = null;

vals ??= new List<int>() {1, 2, 3, 4, 5, 6};
vals.Add(7);
vals.Add(8);
vals.Add(9);

Console.WriteLine(string.Join(", ", vals));

vals ??= new List<int>() {1, 2, 3, 4, 5, 6};

Console.WriteLine(string.Join(", ", vals));

In the example, we use the null-coalescing assignment operator on a list of integer values.

List<int> vals = null;

First, the list is assigned to null.

vals ??= new List<int>() {1, 2, 3, 4, 5, 6};

We use the ??= to assign a new list object to the variable. Since it is null, the list is assigned.

vals.Add(7);
vals.Add(8);
vals.Add(9);

Console.WriteLine(string.Join(", ", vals));

We add some values to the list and print its contents.

vals ??= new List<int>() {1, 2, 3, 4, 5, 6};

We try to assign a new list object to the variable. Since the variable is not null anymore, the list is not assigned.

$ dotnet run
1, 2, 3, 4, 5, 6, 7, 8, 9
1, 2, 3, 4, 5, 6, 7, 8, 9

C# ternary operator

The ternary operator ?: is a conditional operator. It is a convenient operator for cases where we want to pick up one of two values, depending on the conditional expression.

cond-exp ? exp1 : exp2

If cond-exp is true, exp1 is evaluated and the result is returned. If the cond-exp is false, exp2 is evaluated and its result is returned.

using System;

int age = 31;

bool adult = age >= 18 ? true : false;

Console.WriteLine("Adult: {0}", adult);

In most countries the adulthood is based on your age. You are adult if you are older than a certain age. This is a situation for a ternary operator.

bool adult = age >= 18 ? true : false;

First the expression on the right side of the assignment operator is evaluated. The first phase of the ternary operator is the condition expression evaluation. So if the age is greater or equal to 18, the value following the ? character is returned. If not, the value following the : character is returned. The returned value is then assigned to the adult variable.

$ dotnet run
Adult: True

A 31 years old person is adult.

C# Lambda operator

The => token is called the lambda operator. It is an operator taken from functional languages. This operator can make the code shorter and cleaner. On the other hand, understanding the syntax may be tricky. Especially if a programmer never used a functional language before.

Wherever we can use a delegate, we also can use a lambda expression. A definition for a lambda expression is: a lambda expression is an anonymous function that can contain expressions and statements. On the left side we have a group of data and on the right side an expression or a block of statements. These statements are applied on each item of the data.

In lambda expressions we do not have a return keyword. The last statement is automatically returned. And we do not need to specify types for our parameters. The compiler will guess the correct parameter type. This is called type inference.

using System;
using System.Collections.Generic;

var list = new List<int>() { 3, 2, 1, 8, 6, 4, 7, 9, 5 };

var subList = list.FindAll(val => val > 3);

foreach (int i in subList)
{
    Console.WriteLine(i);
}

We have a list of integer numbers. We print all numbers that are greater than 3.

var list = new List<int>() { 3, 2, 1, 8, 6, 4, 7, 9, 5 };

We have a generic list of integers.

var subList = list.FindAll(val => val > 3);

Here we use the lambda operator. The FindAll method takes a predicate as a parameter. A predicate is a special kind of a delegate that returns a boolean value. The predicate is applied for all items of the list. The val is an input parameter specified without a type. We could explicitly specify the type but it is not necessary.

The compiler will expect an int type. The val is a current input value from the list. It is compared if it is greater than 3 and a boolean true or false is returned. Finally, the FindAll will return all values that met the condition. They are assigned to the sublist collection.

foreach (int i in subList)
{
    Console.WriteLine(i);
}

The items of the sublist collection are printed to the terminal.

$ dotnet run
8
6
4
7
9
5

Values from the list of integers that are greater than 3.

using System;
using System.Collections.Generic;

var nums = new List<int>() { 3, 2, 1, 8, 6, 4, 7, 9, 5 };

var nums2 = nums.FindAll( delegate(int i) {
        return i > 3;
    }
);

foreach (int i in nums2)
{
    Console.WriteLine(i);
}

This is the same example. We use a anonymous delegate instead of a lambda expression.

C# calculating prime numbers

We are going to calculate prime numbers.

using System;

int[] nums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 
    14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 };

Console.Write("Prime numbers: ");

foreach (int num in nums)
{
    if (num == 1) continue;

    if (num == 2 || num == 3)
    {
        Console.Write(num + " ");
        continue;
    }

    int i = (int) Math.Sqrt(num);
    bool isPrime = true;

    while (i > 1)
    {
        if (num % i == 0)
        {
            isPrime = false;
        }
        i--;
    }

    if (isPrime)
    {
        Console.Write(num + " ");
    }
}

Console.Write('\n');

In the above example, we deal with many various operators. A prime number (or a prime) is a natural number that has exactly two distinct natural number divisors: 1 and itself. We pick up a number and divide it by numbers, from 1 up to the picked up number. Actually, we do not have to try all smaller numbers; we can divide by numbers up to the square root of the chosen number. The formula will work. We use the remainder division operator.

int[] nums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 
    14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 };

We calculate primes from these numbers.

if (num == 1) continue;

By definition, 1 is not a prime

if (num == 2 || num == 3)
{
    Console.Write(num + " ");
    continue;
}

We skip the calculations for 2 and 3: they are primes. Note the usage of the equality and conditional or operators. The == has a higher precedence than the || operator. So we do not need to use parentheses.

int i = (int) Math.Sqrt(num);

We are OK if we only try numbers smaller than the square root of a number in question. It was mathematically proven that it is sufficient to take into account values up to the square root of the number in question.

while (i > 1)
{
    ...
    i--;
}

This is a while loop. The i is the calculated square root of the number. We use the decrement operator to decrease the i by one each loop cycle. When the i is smaller than 1, we terminate the loop. For example, we have number 9. The square root of 9 is 3. We divide the 9 number by 3 and 2.

if (num % i == 0)
{
    isPrime = false;
}

This is the core of the algorithm. If the remainder division operator returns 0 for any of the i values then the number in question is not a prime.

In this part of the C# tutorial, we covered C# operators.