Default operator in C#

The default operator in C# returns the default value for any data type. For value types, it returns zero or false, while for reference types, it returns null. The default expressions are evaluated at compile-time, making them efficient for initialization.

Starting with C# 7.1, you can use the simplified default literal when the type can be inferred from the context.

Syntax

Following is the syntax for the traditional default operator −

default(Type)

Following is the syntax for the simplified default literal (C# 7.1+) −

Type variable = default;

Default Values for Common Types

Using default() Operator

Example

using System;

public class Demo {
   public static void Main() {
      int val1 = default(int);
      long val2 = default(long);
      bool val3 = default(bool);
      double val4 = default(double);
      string val5 = default(string);
      char val6 = default(char);

      Console.WriteLine("int default: " + val1);
      Console.WriteLine("long default: " + val2);
      Console.WriteLine("bool default: " + val3);
      Console.WriteLine("double default: " + val4);
      Console.WriteLine("string default: " + (val5 ?? "null"));
      Console.WriteLine("char default: '" + val6 + "'");
   }
}

The output of the above code is −

int default: 0
long default: 0
bool default: False
double default: 0
string default: null
char default: ''

Using default Literal (C# 7.1+)

Example

using System;

public class Program {
   public static void Main() {
      // Type inference with default literal
      int number = default;
      bool flag = default;
      string text = default;
      
      Console.WriteLine("number: " + number);
      Console.WriteLine("flag: " + flag);
      Console.WriteLine("text: " + (text ?? "null"));
      
      // Using in method parameters
      ProcessData(default, default);
   }
   
   static void ProcessData(int value, string message) {
      Console.WriteLine($"Processing value: {value}, message: {message ?? "null"}");
   }
}

The output of the above code is −

number: 0
flag: False
text: null
Processing value: 0, message: null

Common Use Cases

• Variable initialization: Initialize variables to their type's default value.

• Generic programming: Useful when working with generic types where the default value is unknown at compile time.

• Optional parameters: Provide default values for method parameters.

• Array initialization: Fill arrays with default values efficiently.

Example with Generic Method

using System;

public class GenericExample {
   public static T GetDefaultValue<T>() {
      return default(T);
   }
   
   public static void Main() {
      Console.WriteLine("Default int: " + GetDefaultValue<int>());
      Console.WriteLine("Default string: " + (GetDefaultValue<string>() ?? "null"));
      Console.WriteLine("Default bool: " + GetDefaultValue<bool>());
   }
}

The output of the above code is −

Default int: 0
Default string: null
Default bool: False

Conclusion

The default operator provides a consistent way to get the default value for any type in C#. It's particularly useful in generic programming and for initializing variables. The simplified default literal introduced in C# 7.1 makes code more concise when the type can be inferred.