Forms Data Controls :: GridView Does Not Contain A Definition For C# Method?

I am trying to create an event with a gridView and a c# method. So far I am getting this error:

CS1061: 'ASP.contentpage9_aspx' does not contain a definition for 'ImageGridView_OnSelectedIndexChanged' and no extension method 'ImageGridView_OnSelectedIndexChanged' accepting a first argument of type 'ASP.contentpage9_aspx' could be found (are you missing a using directive or an assembly reference?)

Here is the asp markup:

<asp:GridView ID="ImageGridView"
DataSourceID="ImageSqlDataSource"
AutoGenerateColumns="false"
DataKeyNames="ImageID"
OnSelectedIndexChanged="ImageGridView_OnSelectedIndexChanged"
RunAt="Server">

And my c# method:

void ImageGridView_OnSelectedIndexChanged(Object sender, EventArgs e)
{
ImageDetailsSqlDataSource.SelectParameters["ImgID"].DefaultValue =
ImageGridView.SelectedValue.ToString();
ImageFormView.DataBind();
}