Use
select * from Tab Where [Part_No] like '%'+@Part_No+'%' and [Company_Name] like '%'+@Company_Name+'%'
or enclose the parameter values with the percentage characters:
SqlCommand sda = new SqlCommand(" select * from Tab Where [Part_No] like @Part_No and [Company_Name] like @Company_Name", con);
sda.Parameters.AddWithValue("Part_No", "%" + TxtSearch.Text + "%");
sda.Parameters.AddWithValue("Company_Name", "%" + TextBox1.Text + "%");