Default

Every reference and value type has a default value. This value is returned by the default(Type) expression. Default is most useful for writing generic classes.

In a generic method, sometimes we need to return a value that is not yet initialized—we can use default() for this.

First example

We look at 4 uses of the default operator. The 4 types we apply the default expression to are the StringBuilder, int, bool and Program types.

using System;
using System.Text;

class Program
{
    static void Main()
    {
        // Acquire the default values for these types and assign to a variable.
        StringBuilder variable1 = default(StringBuilder);
        int variable2 = default(int);
        bool variable3 = default(bool);
        Program variable4 = default(Program);

        // Write the values.
        Console.WriteLine(variable1); // Null
        Console.WriteLine(variable2); // 0
        Console.WriteLine(variable3); // False
        Console.WriteLine(variable4); // Null
    }
}
       (Blank)
0
False
       (Blank)

Default, generic class

Here we have the generic class Test, which requires a class parameter with a public parameterless constructor.

using System;
using System.Text;

class Test<T> where T : class, new()
{
    int _size;

    public T GetOrDefault()
    {
        // Use default in a generic class method.
        return _size == 0 ?
            default(T) :
            new T();
    }
}

class Program
{
    static void Main()
    {
        Test<StringBuilder> test = new Test<StringBuilder>();
        Console.WriteLine(test.GetOrDefault() == null);
    }
}
True

Discussion

The default value expression is implemented using static analysis. This means the default expressions are evaluated at compile-time, resulting in no performance loss.

We looked at the default value expression. By understanding the usage of the default value expression, we can better understand the type system.