@paramResult int,
@paramUsername varchar(25),
@paramPassword varchar(25) as
SET @paramResult = SELECT COUNT(*) FROM tblUser WHERE Username = @paramUsername AND Password = @paramPassword
return @paramResult

using (SqlConnection con = new SqlConnection(pconnStr))
{
        using (SqlCommand cmd = new SqlCommand("CheckLoginCredentials", con))
        {
            cmd.CommandType = CommandType.StoredProcedure;

            cmd.Parameters.Add("@paramUsername", SqlDbType.VarChar).Value = txtUsername.Text.Trim();
            cmd.Parameters.Add("@paramPassword", SqlDbType.VarChar).Value = txtPassword.Text.Trim();

            var returnParameter = cmd.Parameters.Add("@paramResult", SqlDbType.Int);

            con.Open();
            cmd.ExecuteNonQuery();
            var result = returnParameter.Value;
        }
}

DataTable ldtIfUserExists = new DataTable();
result.Fill(ldtIfUserExists);