有没有办法指定“空” C# lambda 表达式？

Question

I'd like to declare an "empty" lambda expression that does, well, nothing. Is there a way to do something like this without needing the DoNothing() method?

public MyViewModel()
{
    SomeMenuCommand = new RelayCommand(
            x => DoNothing(),
            x => CanSomeMenuCommandExecute());
}

private void DoNothing()
{
}

private bool CanSomeMenuCommandExecute()
{
    // this depends on my mood
}

My intent in doing this is only control the enabled/disabled state of my WPF command, but that's an aside. Maybe it's just too early in the morning for me, but I imagine there must be a way to just declare the x => DoNothing() lambda expression in some way like this to accomplish the same thing:

SomeMenuCommand = new RelayCommand(
    x => (),
    x => CanSomeMenuCommandExecute());

Is there some way to do this? It just seems unnecessary to need a do-nothing method.

Answer 1

Action doNothing = () => { };

Answer 2

I thought I would add some code that I've found useful for this type of situation. I have an Actions static class and a Functions static class with some basic functions in them:

public static class Actions
{
  public static void Empty() { }
  public static void Empty<T>(T value) { }
  public static void Empty<T1, T2>(T1 value1, T2 value2) { }
  /* Put as many overloads as you want */
}

public static class Functions
{
  public static T Identity<T>(T value) { return value; }

  public static T0 Default<T0>() { return default(T0); }
  public static T0 Default<T1, T0>(T1 value1) { return default(T0); }
  /* Put as many overloads as you want */

  /* Some other potential methods */
  public static bool IsNull<T>(T entity) where T : class { return entity == null; }
  public static bool IsNonNull<T>(T entity) where T : class { return entity != null; }

  /* Put as many overloads for True and False as you want */
  public static bool True<T>(T entity) { return true; }
  public static bool False<T>(T entity) { return false; }
}

I believe this helps improve readability just a tiny bit:

SomeMenuCommand = new RelayCommand(
        Actions.Empty,
        x => CanSomeMenuCommandExecute());

// Another example:
var lOrderedStrings = GetCollectionOfStrings().OrderBy(Functions.Identity);

Answer 3

This should work:

SomeMenuCommand = new RelayCommand(
    x => {},
    x => CanSomeMenuCommandExecute());

Answer 4

Assuming you only need a delegate (rather than an expression tree) then this should work:

SomeMenuCommand = new RelayCommand(
        x => {},
        x => CanSomeMenuCommandExecute());

(That won't work with expression trees as it's got a statement body . See section 4.6 of the C# 3.0 spec for more details.)

Answer 5

I don't fully understand why do you need a DoNothing method.

Can't you just do:

SomeMenuCommand = new RelayCommand(
                null,
                x => CanSomeMenuCommandExecute());

Answer 6

Action DoNothing = delegate { };
Action DoNothing2 = () => {};

I used to initialize Events to a do nothing action so it was not null and if it was called without subscription it would default to the 'do nothing function' instead of a null-pointer exception.

public event EventHandler<MyHandlerInfo> MyHandlerInfo = delegate { };

Answer 7

Starting with C# 9.0 you can specify discards _ for required parameters. Example:

Action<int, string, DateTime> action = (_, _, _) => { };