Wednesday, 18 March 2009

Using Generics With System.Nullable<T>

When we have a Nullable type value and we want to assign it to some control, then we need to check if is not null first.

SomeLabel.Text = object.MyProperty.HasValue ? object.MyProperty.Value : String.Empty;

But if we have many nullable properties? I don't want to copy/paste this check on every line. So we need a method, that checks for nulls. But I also don't want several methods for every type. So we need to use generics.

I created some methods for myself

public static string ValueOrEmptyString<T>(Nullable<T> obj) where T : struct
  return obj.HasValue ? obj.Value.ToString() : "";

public static T ValueOrDefault<T>(Nullable<T> obj, T defaultValue) where T : struct
  return obj.HasValue ? obj.Value : defaultValue;

public static T ValueOrDefault<T>(Nullable<T> obj) where T : struct
  return obj.HasValue ? obj.Value : default(T);

"where T : struct" - is needed for compiler to understand that this method is allowed only for types that belong to System.ValueType.

I can use these methods like that

SomeLabel.Text = ValueOrEmptyString<int>( object.MyIntProperty );
SomeDateTimeControl.Date = ValueOrDefault<DateTime>( object.MyDateTimeProperty );
SomeReferenceControl.SelectedObjecId = ValueOrDefault<int>( object.MyIntProperty, -1 );