Getting return value from dynamic sql from stored procedure (SQL Server and C# ADO.NET)?

Question

I created a stored procedure to get return value from a dynamic sql. I get the following exception:

String[1]: the Size property has an invalid size of 0.

My proc:

CREATE proc [dbo].[Review_Get_PrePopValue](@sqlQuery nvarchar(500), @display nvarchar(200) OUTPUT)
as

EXEC sp_executesql @sqlQuery,   
    @display OUTPUT 

My Code:

public string GetAnswerValue(string contentSQL, string parameter, string parameterValue)
{
    string sqlstatement = contentSQL.Replace(parameter, parameterValue);

    using (var conn = new SqlConnection(_connectionString))
    {
        string prePopValue;
        SqlCommand comm = new SqlCommand();
        comm.Connection = conn;
        comm.CommandType = CommandType.StoredProcedure;
        comm.CommandText = "Review_Get_PrePopValue";

        comm.Parameters.Add(new SqlParameter("@sqlQuery", SqlDbType.NVarChar)).Value = sqlstatement; 
        comm.Parameters.Add(new SqlParameter
        {
            Direction = ParameterDirection.Output,
            ParameterName = "@display",
            SqlDbType = SqlDbType.NVarChar
        });

        conn.Open();
        comm.ExecuteNonQuery();

        prePopValue = comm.Parameters["@display"].Value.ToString();
        return prePopValue;
    }
}

My sql string:

Select @display = Grant_Number From GMIS_Grants where grant_id=1

Answer 1

Direction = ParameterDirection.Output, ParameterName = "@display", Size = 200, SqlDbType = SqlDbType.NVarChar

Needed to set the size for nvarchar

Answer 2

From MSDN

For bidirectional and output parameters, and return values, you must set the value of Size.

Also, add this at the end of your SP.

SELECT @display;