Cannot convert lambda expression to type 'obje

问题:

I have a base class that has a bool property which looks like this:

public abstract class MyBaseClass
{
     public bool InProgress { get; protected set; }
}

I am inheriting it another class from it and trying to add InProgress as a delegate to the dictionary. But it throws me an error. This is how my Derived class looks like:

public abstract class MyClass
{
     Dictionary<string, object> dict = new Dictionary<string, object>();
     dict.Add("InProgress", InProgress => base.InProgress = InProgress);

}

This is the error I am getting:

Cannot convert lambda expression to type 'object' because it is not a delegate type

What am I doing wrong here?

回答1:

Best would be to have the dictionary strongly typed, but if you assign the lambda to a specific lambda (delegate) first, it should work (because the compiler then knows the delegate format):

        Action<bool> inp = InProgress => base.InProgress = InProgress;
        dict.Add("InProgress", inp);

Or by casting it directly, same effect

       dict.Add("InProgress", (Action<bool>)(InProgress => base.InProgress = InProgress));

Of course having such a dictionary format as object is discussable, since you'll have to know the delegate format to be able to use it.

回答2:

I got this error when I was missing

using System.Data.Entity;

回答3:

Although the solution by @Me.Name is completely valid by itself, there's an additional trick that may come in handy in some situations (it certainly did for me): if you're converting multiple lambdas using this technique, you can factor the cast as a helper method, along the lines of

object myDelegateToObject ( Action<bool> action ) {
    return action; // autocast to `object` superclass, no explicit cast needed
}

and then call it by simply

dict.Add("InProgress", myDelegateToObject(InProgress => base.InProgress = InProgress));

It may save you time later on - if you decide to change to change the signatures, you will have to do so in one place only.

回答4:

I ran into this problem while writing unit tests. I was attempting to mock the behavior of a database to return a new object from a repository instead of actually connecting to a database.

Make sure your object has a usable constructor. You may not be able to successfully instantiate that object the way you want to. Ensure if you using a lambda to point to a constructor that the constructor can be called in the same way in a normal instantiation statement.

i.e.

return x => new FakeObject();

say in the case of

var fake = new FakeObject();

would not work then the lambda will also fail so be careful.