拖放用户控件将无法在.net windows项目中工作

Question

I created a user control and it shows up on the tool box as form components. Then when I try to drag and drop the user control on to a form , I get this visual studio error.

" The specified named connection is either not found in the configuration ,not intended to be used with the entity client provider or not valid."

Why am I getting this error?

But some other user controls I can drag and drop which are under the same project. I don't know what I missed in creating this user control.

Answer 1

Beware that code in the UserControl class runs at design time. The constructor, the OnLoad method and Load event. But also methods like OnPaint(). If this code does anything that depends on the environment being setup properly, that code is liable to throw an exception and cause the designer to change its mind about adding the control to the form. That certainly seems to be the case when you get a "not found in the configuration" error, there is no configuration file yet.

Use the DesignMode properly to skip such code. Like this:

    protected override void OnLoad(EventArgs e) {
        if (!this.DesignMode) {
            // Do stuff...
        }
        base.OnLoad(e);
    }

Answer 2

this error show if you put the code of loading data from database into the constructor of userControl.

"loading data or initialize entity framework"

so the solution is to move the code of loading data from constructor to a method. you can call it "loadData".

and call this method "loadData" in the constructor of the parent form

Answer 3

As Hans says, you might need to use the DesignMode property in the Constructor or OnLoad. Also, make sure any public properties that use the connection have this attribute:

[DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
public string Foo
{
    get;
    set;
}

That way, the designer won't attempt to set them when you add the control to a form. This is always a good habit to get into anyway for properties that you won't be setting at design time.