Try:
cmd.CommandText = "SELECT FirstName + ' ' + LastName, CompanyName, Department, Location, MobileNo from ResidentTable where EmployeeNo='" + TxtSearch.Text + "'";
But please, don't do that! Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
cmd.CommandText = "SELECT FirstName + ' ' + LastName, CompanyName, Department, Location, MobileNo from ResidentTable where EmployeeNo=@EN";
cmd.Parameters.AddWithValue("@EN", TxtSearch.Text);