Skip to main content

Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type

Last week I was trying to pass a lambda as a parameter to a method that expected a delegate. However doing this resulted in the following compiler error

“Cannot convert lambda expression to type 'System.Delegate' because it is not a delegate type.”    

   1:  public void CreateAndExecuteLambda()
   2:  {
   3:     //Does not compile
   4:     InvokeDelegate(() => MessageBox.Show("Hello World")); 
   5:  }
   6:   
   7:  public void InvokeDelegate(Delegate @delegate) 
   8:  {
   9:     @delegate.DynamicInvoke();
  10:   }

At first I found this very strange, if I remembered it well a lambda expression is just a new syntax for anonymous methods and anonymous methods are nothing more than inline delegates. So why the compiler does not agree?

The reason is that a lambda expression can either be converted to a delegate type or an expression tree - but it has to know which delegate type. Just knowing the signature isn't enough. For instance, suppose I have:

   1:  public delegate void Action1();
   2:  public delegate void Action2();   
   3:   
   4:  Delegate x = () => Console.WriteLine("hello world");

What would you expect the concrete type of the object referred to by x to be?

If you want to make it work you have to help the compiler by explicitly casting the lambda expression to the correct delegate type. 

   1:  public void CreateAndExecuteLambda()
   2:  {
   3:     //This works!
   4:     InvokeDelegate((Action)(() => MessageBox.Show("Hello World"))); 
   5:   }
   6:   
   7:  public void InvokeDelegate(Delegate @delegate)
   8:  {
   9:     @delegate.DynamicInvoke();
  10:  }
Labels:

Popular posts from this blog

Podman– Command execution failed with exit code 125

After updating WSL on one of the developer machines, Podman failed to work. When we took a look through Podman Desktop, we noticed that Podman had stopped running and returned the following error message: Error: Command execution failed with exit code 125 Here are the steps we tried to fix the issue: We started by running podman info to get some extra details on what could be wrong: >podman info OS: windows/amd64 provider: wsl version: 5.3.1 Cannot connect to Podman. Please verify your connection to the Linux system using `podman system connection list`, or try `podman machine init` and `podman machine start` to manage a new Linux VM Error: unable to connect to Podman socket: failed to connect: dial tcp 127.0.0.1:2655: connectex: No connection could be made because the target machine actively refused it. That makes sense as the podman VM was not running. Let’s check the VM: >podman machine list NAME         ...
Read more

Azure DevOps/ GitHub emoji

I’m really bad at remembering emoji’s. So here is cheat sheet with all emoji’s that can be used in tools that support the github emoji markdown markup: All credits go to rcaviers who created this list.
Read more

VS Code Planning mode

After the introduction of Plan mode in Visual Studio , it now also found its way into VS Code. Planning mode, or as I like to call it 'Hannibal mode', extends GitHub Copilot's Agent Mode capabilities to handle larger, multi-step coding tasks with a structured approach. Instead of jumping straight into code generation, Planning mode creates a detailed execution plan. If you want more details, have a look at my previous post . Putting plan mode into action VS Code takes a different approach compared to Visual Studio when using plan mode. Instead of a configuration setting that you can activate but have limited control over, planning is available as a separate chat mode/agent: I like this approach better than how Visual Studio does it as you have explicit control when plan mode is activated. Instead of immediately diving into execution, the plan agent creates a plan and asks some follow up questions: You can further edit the plan by clicking on ‘Open in Editor’: ...
Read more