Method with a generic parameter

Question

User-1136635841 posted

Hi there

I have these two methods that return the Text of some controls on my GridView :

private string GetLiteralValue(string controlId)
{
  Literal control = (Literal)myGridView.SelectedRow.FindControl(controlId);
  return control.Text;
}

 

private string GetTextBoxValue(string controlId)
{
  TextBox textbox = (TextBox)myGridView.SelectedRow.FindControl(controlId);
  return textbox.Text;
}

Instead, I would like to have a generic method that would look for the type of control I specify when calling it. Something like this :

private string GetControlValue<T>(string controlId)
{
  T control = (T)myGridView.SelectedRow.FindControl(controlId);
  return control.Text;
}

This won't compile however, since the T type does not have a Text property. I also tried this :

private string GetControlValue<T>(string controlId) where T : WebControl

But it's not working either.

Is there something that I forgot to add in my method, so that the compiler knows that my T type can be any WebControl with a Text property ?

Thanks

Answer 1

User437720957 posted

private string GetControlValue<T>(string controlId) where T : System.Web.UI.ITextControl

 

Answer 2

User1187105292 posted

in the where clause that goes with T, you can specify that T implements a given class and/or that it implements one or more interfaces.

The trick is to find one class or interface that guarantees the Text property.

By "Going to Definition" on a Textbox and a DropDownList (and working up the chain of super-classes for DropDownList), I found both of these controls implement the ITextControl interface. 

This interface looks like this:

using System;

namespace System.Web.UI

{

// Summary:

// Defines the interface a control implements to get or set its text content.

public interface ITextControl

{

// Summary:

// Gets or sets the text content of a control.

//

// Returns:

// The text content of a control.

string Text { get; set; }

}

}

 So, specify that T must implement this interface and I think you'll be in good shape.

 

Answer 3

User1187105292 posted

Forgot to add that when you don't get lucky like that, you can always subclass a control and have your subclass(es) implement the interface you need (which  you also might have just created).  Then drag the subclass onto the page instead of TextBox, etc.

Answer 4

User-1136635841 posted

Hi guys

Thanks for you answers.

I tried with ITextControl, then I got this compilation error :

Cannot convert type 'System.Web.UI.Control' to 'T' 

So I also added the WebControl class in the "where" clause :

private string GetControlValue<T>(string controlId) where T : WebControl, ITextControl

Now my code compiles, it works when T is a TextBox or a DropDownList, however it doesn't work with Literal controls : if I try

string name = GetControlValue<Literal>("ltrName");

I get the following error :

The type 'System.Web.UI.WebControls.Literal' must be convertible to 'System.Web.UI.WebControls.WebControl' in order to use it as parameter 'T' in the generic type or method 'GetControlValue<T>(string)'

That doesn't make sense to me, isn't Literal a WebControl ? I also tried with the LiteralControl class but I get the same error

Answer 5

User437720957 posted

That doesn't make sense to me, isn't Literal a WebControl ?

Nope.  Literal only inherits from Control. But, you won't need that part anyway....

 

 

private string GetControlValue<T>(string controlId) where T : ITextControl
{
    ITextControl ctrl = myGridView.SelectedRow.FindControl(controlId) as ITextControl;
    if (ctrl!=null)
        return ctrl.Text;
    return string.Empty;
}</T>

 

Answer 6

User312496708 posted

If that is your requirement then you class decleration should be

private string GetControlValue<T>(string controlId) where T :  ITextControl

You cannot allow T to be any control but only those which implements ITextControl interface

Answer 7

User-1136635841 posted

Hello

I actually changed my method so that it returns the control itselft instead of just its content (because sometimes I need to use the SelectedValue and DataSource properties of the controls I retrieve)

Since Literal inherits from Control I changed the where clause the following way :

private T GetControl<T>(string controlId) where T : Control, ITextControl

So now it works with every kind of control

Thanks for you help everyone !