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C# Default

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. It also helps us understand the language's type system.


Example. We first look at four uses of the default expression. The four types we apply the default expression to are the StringBuilder, int, bool and Program types. The Program type is declared in the program itself.

IntBool

Finally: The values that were assigned are printed to the screen. The null value is printed as a blank line.

Null
C# program that uses default expression

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
    }
}

Output

       (Blank)
0
False
       (Blank)


Discussion. At the level of the intermediate language instructions, the default value expression is implemented using static analysis. This means the default expressions are evaluated at compile-time, resulting in no performance loss.

So: No reflection to the type system or metadata relational database is used at runtime.

IL

And: Unlike with typeof expressions, caching the default expression would have no benefit.

Typeof

Summary. We looked at the default value expression. The ECMA-344 specification describes this expression in chapter 14 and page 187. By understanding the usage of the default value expression, we can better understand the type system.