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When you have your own custom TemplateControl and perform a LoadControl in there, this results in ax exception.

For instance, the following code will trigger the exception:

public class NavController : System.Web.UI.TemplateControl
{
    public void LoadContent(string url)
    {
        Control ctrl = this.LoadControl(url);

        this.Controls.Clear();
        this.Controls.Add(ctrl);
    }
}

The exception you get is:

[ArgumentNullException: Value cannot be null.
Parameter name: basepath]
   System.Web.Util.UrlPath.Combine(String appPath, String basepath, String relative) +322
   System.Web.UI.TemplateControl.LoadControl(String virtualPath) +29
   SimpleContainer.NavController.LoadContent(String url)

The reason for this is that the property AppRelativeTemplateSourceDirectory is not correctly initialized. The workaround for this is to add the following code to your TemplateControl:

protected override void OnInit(EventArgs e)
{
    this.AppRelativeTemplateSourceDirectory = Page.AppRelativeTemplateSourceDirectory;
    base.OnInit(e);
}