Excel VSTO add-in won’t show my Windows Form

27 April 2016

You put the Windows Form in a separate project because you were trying to be a good engineer, didn’t you?

Excel can’t load the dependency DLL so it can’t/won’t display your form. Move your form into the same project as the add-in (You could create a GUI or Forms folder with its own namespace)

Example

These are example event handlers on Ribbon buttons.

Simple MessageBox - works fine

private void goButton_Click(object sender, RibbonControlEventArgs e)
{
	MessageBox.Show("Hello world");
}

Create an anonymous form and display it - works fine

private void goButton_Click(object sender, RibbonControlEventArgs e)
{
	var win = new Form();
	win.Text = "New Form";
	win.Size = new Size(600, 400);
	win.Location = new System.Drawing.Point(100,100);
	win.Show();
}

Display a form from another project - won’t even hit breakpoint on first line

private void goButton_Click(object sender, RibbonControlEventArgs e)
{
	string junk = "Excel will not execute any part of this method. Not even this line.";
	
	//Namespace used to illustrate that it's in a different project
	var setupForm = new GuiProject.SetupForm();
    setupForm.Show();
}

Display a form from same project - should work

private void goButton_Click(object sender, RibbonControlEventArgs e)
{
	//SetupForm can be in its own namespace as long as it's in this project! :)
	var setupForm = new GUI.SetupForm();
    setupForm.Show();
}