How to return value from Action ()?

Question

As for the answer to this question Passing a DataContext to Action () , how do I return a value from an action (db)?

SimpleUsing.DoUsing(db => { // do whatever with db }); 

It should be more like:

 MyType myType = SimpleUsing.DoUsing<MyType>(db => { // do whatever with db. query buit using db returns MyType. }); 

Answer 1

Your static method should go from:

 public static class SimpleUsing { public static void DoUsing(Action<MyDataContext> action) { using (MyDataContext db = new MyDataContext()) action(db); } } 

To:

 public static class SimpleUsing { public static TResult DoUsing<TResult>(Func<MyDataContext, TResult> action) { using (MyDataContext db = new MyDataContext()) return action(db); } } 

This answer grew out of comments so I can provide the code. For full development, see @sll answer below.

Answer 2

You can use the generic delegate Func<T, TResult> . (See MSDN )

 Func<MyType, ReturnType> func = (db) => { return new MyType(); } 

There are also useful generic delegates that take into account the return value:

• Converter<TInput, TOutput> ( MSDN )
• Predicate<TInput> - always return bool ( MSDN )

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Method:

 public MyType SimpleUsing.DoUsing<MyType>(Func<TInput, MyType> myTypeFactory) 

General delegate:

 Func<InputArgumentType, MyType> createInstance = db => return new MyType(); 

Performance:

 MyType myTypeInstance = SimpleUsing.DoUsing( createInstance(new InputArgumentType())); 

OR explicitly:

 MyType myTypeInstance = SimpleUsing.DoUsing(db => return new MyType()); 

Answer 3

You can also take advantage of the fact that a lambda or anonymous method can close variables in its scope.

 MyType result; SimpleUsing.DoUsing(db => { result = db.SomeQuery(); //whatever returns the MyType result }); //do something with result 

Answer 4

Use Func<T> instead of Action<T> .

Action<T> acts like a void method with a parameter of type T, while Func<T> works like a function without parameters and returns an object of type T. If you want to set the parameters of your function, use Func<TReturn, TParameter> .