Generics, Dictionaries, and More - 4.12 Initializing Generic Variables to Their Default Values

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Problem

You have a generic class that contains a variable of the same type as the type parameter defined by the class itself. Upon construction of your generic object, you want that variable to be initialized to its default value.

Solution

Simply use the default keyword to initialize that variable to its default value:

  public class DefaultValueExample<T>
  {
     T data = default(T);

     public bool IsDefaultData()
     {
        T temp = default(T);

        if (temp.Equals(data))
        {
           return (true);
        }
        else
        {
           return (false);
        }
     }

     public void SetData(T val)
     {
        data = val;
     }
  }

The code to use this class is shown here:

  public static void ShowSettingFieldsToDefaults()
  {
     DefaultValueExample<int> dv = new DefaultValueExample<int>();

     // Check if the data is set to its default value; true is returned.
     bool isDefault = dv.IsDefaultData();
     Console.WriteLine("Initial data: " + isDefault);

     // Set data.
     dv.SetData(100);
     // Check again, this time a false is returned.
     isDefault = dv.IsDefaultData();
     Console.WriteLine("Set data: " + isDefault);
  }

The first call toIsDefaultDatareturnstrue, while the second returnsfalse. The output is shown here:

  Initial data: True
  Set data: False

Discussion

When initializing a variable of the same type parameter as the generic class, you cannot just set that variable to null. What if the type parameter is a value type such as an int or char? This will not work because value types cannot be null. You may be thinking that anullabletype such aslong?orNullable<long>can be set tonull(see Recipe 4.6 for more on nullable types). However, the compiler has no way of knowing what type argument the user will use to construct the type.

Thedefaultkeyword allows you to tell the compiler that at compile time the default value of this variable should be used. If the type argument supplied is a numeric value (e.g.,int,long,decimal), then the default value is zero. If the type argument supplied is a reference type, then the default value isnull. If the type argument supplied is astruct, then the default value of thestruct is determined by initializing each member field to its default value.

See Also

Recipe 4.6, and the “default Keyword in Generic Code” topic in the MSDN documentation.

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