stored procedure in linq to search

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SQL-Server

I use combobox in my form, and add items for example fristName, lastName, age
I want when user one of this item and type in textbox expression,fill
datagridview result query

my stored procedure:

ALTER PROCEDURE dbo.StoredProcedure7
	@orderby as nvarchar(10),@searchText as nvarchar(50)

AS
begin
	select * from Table2
WHERE
 @searchText= 
  CASE @orderby
      WHEN 'frirstName' THEN id2
      WHEN 'lastName' THEN id1 
      WHEN 'age' THEN id3 
  END

end

but this procedure has error and doesn't show anything in datagrid


when i chane nvarchar(max) to nvarchar(50) and use this code to my form:
dataGridView1.DataSource = dc.StoredProcedure8(comboBox1.SelectedItem.ToString(), texbox.Text);

Conversion failed when converting the nvarchar value 'dd' to data type int.