Tutorial :Cannot convert type 'System.Windows.Forms.Control' to 'T'

Question:

I am trying to make a Generic FindControl method and I get the following error:

Cannot convert type 'System.Windows.Forms.Control' to 'T'

Code:

public T Control<T>(String id)  {    foreach (Control ctrl in MainForm.Controls.Find(id, true))    {      return (T)ctrl; // Form Controls have unique names, so no more iterations needed    }      throw new Exception("Control not found!");  }

Solution:1

try this

public T Control<T>(String id) where T : Control  {    foreach (Control ctrl in MainForm.Controls.Find(id, true))    {      return (T)ctrl; // Form Controls have unique names, so no more iterations needed    }      throw new Exception("Control not found!");  }

Solution:2

You could always bend the rules and do a double cast. Eg:

public T Control<T>(String id)  {    foreach (Control ctrl in MainForm.Controls.Find(id, true))    {      return (T)(object)ctrl;     }      throw new Exception("Control not found!");  }

Solution:3

As T is unconstrained, you could pass anything for the type parameter. You should add a 'where' constraint to your method signature:

  public T Control<T>(string id) where T : Control  {      ...  }

Solution:4

How are you calling this method, have you got an example?

Also, I'd add a constraint to your method:

public T Control<T>(string id) where T : System.Windows.Forms.Control  {      //  }

Solution:5

While other people have already correctly pointed out what the problem is, I just want to chime in that this would be quite suitable for an extension method. Don't upvote this, this is actually a comment, I'm just posting it as an answer so that I can gain the ability to write longer and format my code better ;)

public static class Extensions  {     public static T FindControl<T>(this Control parent, string id) where T : Control     {        return item.FindControl(id) as T;     }  }

So that you can invoke it like so:

Label myLabel = MainForm.Controls.FindControl<Label>("myLabelID");

Solution:6

Change your method signature to this:

public T Control<T>(String id) where T : Control

Stating that all T's are in fact of type Control. This will constrain T and the compiler knows you can return it as T.

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