Changing SelectMethod In SQLDataSource

I have a gridview bound to an ObjectDataSource, which is a dataset populated by SQL queries.  The gridview works with the default SelectMethod configured in the ObjectDataSource.  I would like to be able to change the SelectMethod based on user input, which would call a different query.  I have one textbox for input and several buttons to pick the type of filter. 

My gridview populates on with the default selectmethod if I create a button that does nothing but a postback.  My other buttons run the following codes which returns nothing.  Any ideas what I'm doing wrong?

If e.CommandArgument = "Last_Name" Then

EmployeeData.SelectMethod = "GetDataByLastName"

End If

If e.CommandArgument = "First_Name" Then

EmployeeData.SelectMethod = "GetDataByFirstName"

End If

gvPhoto.DataSourceID = EmployeeData.ID

gvPhoto.DataBind()

 

Thanks.

Lisa