What is 'default' keyword in C#?

In C#, the default keyword is used to retrieve the default value of a type. This is particularly useful when working with reference types or nullable value types.

Default values for reference types

For reference types, the default value is null. For example:

string str = default; // str is set to null

Default values for value types

For value types, the default value is the default value of the type's underlying data type. For example:

int i = default; // i is set to 0
double d = default; // d is set to 0.0
bool b = default; // b is set to false

Default values for nullable value types

For nullable value types, the default value is null. For example:

int? i = default; // i is set to null
double? d = default; // d is set to null
bool? b = default; // b is set to null

Using default with generic types

The default keyword can also be used with generic types. For example:

T DefaultValue<T>()
{
    return default;
}

int i = DefaultValue<int>(); // i is set to 0
string str = DefaultValue<string>(); // str is set to null

Using default in conditional statements

The default keyword can also be used in conditional statements, such as a switch statement. For example:

int i = 3;

switch (i)
{
    case 1:
        Console.WriteLine("Case 1");
        break;
    case 2:
        Console.WriteLine("Case 2");
        break;
    default:
        Console.WriteLine("Default case");
        break;
}

In this example, the default case will be executed because the value of i does not match any of the other cases.

Conclusion

The default keyword is a useful tool in C# for retrieving the default value of a type. It can be used with reference types, value types, nullable value types, and generic types. It is also commonly used in conditional statements to specify a default case.