default value expressions - produce the default value
A default value expression produces the default value of a type. There are two kinds of default value expressions: the default operator call and a default literal.
You also use the default keyword as the default case label within a switch statement.
default operator
The argument to the default operator must be the name of a type or a type parameter, as the following example shows:
Console.WriteLine(default(int)); // output: 0
Console.WriteLine(default(object) is null); // output: True
void DisplayDefaultOf<T>()
{
var val = default(T);
Console.WriteLine($"Default value of {typeof(T)} is {(val == null ? "null" : val.ToString())}.");
}
DisplayDefaultOf<int?>();
DisplayDefaultOf<System.Numerics.Complex>();
DisplayDefaultOf<System.Collections.Generic.List<int>>();
// Output:
// Default value of System.Nullable`1[System.Int32] is null.
// Default value of System.Numerics.Complex is (0, 0).
// Default value of System.Collections.Generic.List`1[System.Int32] is null.
default literal
You can use the default literal to produce the default value of a type when the compiler can infer the expression type. The default literal expression produces the same value as the default(T) expression where T is the inferred type. You can use the default literal in any of the following cases:
- In the assignment or initialization of a variable.
- In the declaration of the default value for an optional method parameter.
- In a method call to provide an argument value.
- In a
returnstatement or as an expression in an expression-bodied member.
The following example shows the usage of the default literal:
T[] InitializeArray<T>(int length, T initialValue = default)
{
if (length < 0)
{
throw new ArgumentOutOfRangeException(nameof(length), "Array length must be nonnegative.");
}
var array = new T[length];
for (var i = 0; i < length; i++)
{
array[i] = initialValue;
}
return array;
}
void Display<T>(T[] values) => Console.WriteLine($"[ {string.Join(", ", values)} ]");
Display(InitializeArray<int>(3)); // output: [ 0, 0, 0 ]
Display(InitializeArray<bool>(4, default)); // output: [ False, False, False, False ]
System.Numerics.Complex fillValue = default;
Display(InitializeArray(3, fillValue)); // output: [ (0, 0), (0, 0), (0, 0) ]
Tip
Use .NET style rule IDE0034 to specify a preference on the use of the default literal in your codebase.
C# language specification
For more information, see the Default value expressions section of the C# language specification.
