An if statement identifies which block code to run based on the value of a Boolean expression (Docs, 2015)

bool condition = true;

if (condition)
{
    Console.WriteLine("The variable is set to true.");
}
else
{
    Console.WriteLine("The variable is set to false.");
}

Output:

The variable is set to true.

If the condition is set to false:

bool condition = false;

if (condition)
{
    Console.WriteLine("The variable is set to true.");
}
else
{
    Console.WriteLine("The variable is set to false.");
}

Output:

The variable is set to false.

Live-code example

The else part of the if-else statement is optional:

bool condition = true;

if (condition)
{
    Console.WriteLine("The variable is set to true.");
}
Console.WriteLine("This will always be printed.");

Output:

The variable is set to true.
This will always be printed.

Nested if-else

If-else statements can be nested as well in which multiple conditions can be tested.

bool condition1 = true;
bool condition2 = false;

if (condition1)
{
    if (condition2)
    {
        Console.WriteLine("condition1 and condition2 is true");
    }
    else
    {
        Console.WriteLine("condition1 is true but condition2 is false");
    }
}
else
{
    Console.WriteLine("condition1 is false");
}

Live-code example

Here is another example that test whether the user input is a letter, then check whether it is uppercase or lowercase:

Console.Write("Enter a character: ");
char c = (char)Console.ReadLine()[0];
if (Char.IsLetter(c))
{
    if (Char.IsLower(c))
    {
        Console.WriteLine("The character is lowercase.");
    }
    else
    {
        Console.WriteLine("The character is uppercase.");
    }
}
else
{
    Console.WriteLine("The character isn't an alphabetic character.");
}

Sample output (try it on the live-code example below!):

Enter a character: a
The character is lowercase.
Enter a character: A
The character is uppercase.
Enter a character: 1
The character isn't an alphabetic character.

Live-code example

else if

Other than nesting if-else statements, the else if clause can be used to test multiple condition. The same example above can be written as:

Console.Write("Enter a character: ");
char c = (char)Console.ReadLine()[0];
if (Char.IsLower(c))
{
    Console.WriteLine("The character is lowercase.");
}
else if (Char.IsUpper(c))
{
    Console.WriteLine("The character is uppercase.");
}
else if (Char.IsDigit(c))
{
    Console.WriteLine("The character is a digit");
}
else
{
    Console.WriteLine("The character isn't an alphabetic character.");
}

Sample output:

Enter a character: a
The character is lowercase.
Enter a character: B
The character is uppercase.
Enter a character: 1
The character is a digit
Enter a character: )
The character isn't an alphabetic character.

Live-code example

Logical operators

As we have covered earlier, we can use logical operators to make a compound condition.

NOT

// Logical NOT, !
bool result = true;
if (!result)
{
    Console.WriteLine("result is not true");
}
else
{
    Console.WriteLine("result is not false");
}

AND

// AND, &&
int m = 9;
int n = 7;
int p = 5;
if (m >= n && m >= p)
{
    Console.WriteLine("Nothing is larger than m.");
}

OR

// OR, ||
if (m > n || m > p)
{
    Console.WriteLine("m isn't the smallest.");
}

Combined

// AND and NOT
if (m >= n && !(p > m))
{
    Console.WriteLine("Nothing is larger than m.");
}

// NOT and OR
m = 4;
if (!(m >= n || m >= p))
{
    Console.WriteLine("Now m is the smallest.");
}

Live-code example

References

1. Docs, M. (2015, July 20). if-else (C# Reference). Retrieved from https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/if-else