A default literal expression produces the default value of a type. The default keyword is not a new one, it is being used for a long time. However, some enhancement has been done for default literal expression.
Before C# 7.1, we were able to use the default keyword as default(T) where T can be a value type or reference type.
int intValue = default(int);
double doubleValue = default(double);
bool boolValue = default(bool);
string str = default(string);
int? nullableInt = default(int?);
Action<int, bool> action = default(Action<int, bool>);
Predicate<string> predicate = default(Predicate<string>);
List<string> list = default(List<string>);
Dictionary<int, string> dictionary = default(Dictionary<int, string>);
In C# 7.1, the default value expressions are enhanced by removing the need to pass T as a parameter while finding the default value of type.
double doubleValue = default;
bool boolValue = default;
string str = default;
int? nullableInt = default;
Action<int, bool> action = default;
Predicate<string> predicate = default;
List<string> list = default;
Dictionary<int, string> dictionary = default;
The Type is now inferred rather than passed as an argument. The default literal expressions also work with method arguments and return values.
public int Add(int x, int y = default, int z = default)
{
return x + y + z;
}
default literal expression produces the same value as the default(T) expression where T is the inferred type.