Can't Call Stored Procedure In SqlFunction

There is a SqlFunction in my SQL Server Project:

[Microsoft.SqlServer.Server.SqlFunction(DataAccess=DataAccessKind.Read)]
public static SqlBoolean GetRelation(SqlGuid ID)
{
using (SqlConnection conn = new SqlConnection("context connection=true"))
{
conn.Open();
SqlCommand cmd = new SqlCommand("SELECT * FROM MemberRelation WHERE ID='" + ID + "'", conn);
//SqlCommand cmd = new SqlCommand("EXEC GetMemberRelation '" + ID + "'", conn);
/*SqlCommand cmd = new SqlCommand("GetMemberRelation", conn);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add(new SqlParameter("@ID", SqlDbType.UniqueIdentifier));
cmd.Parameters["@ID"].Value = ID;*/

using (SqlDataReader sdr = cmd.ExecuteReader())
{
sdr.Read();
//...
sdr.Close();
}
conn.Close();
}
}

When I try to call a stored procedure by using one of the two marked code segments instead of concatenating string in GetRelation, I can deploy the project to my SQL server without problem. But when I call GetRelation from Query Analyzer, I get an error: "Invalid use of side-effecting or time-dependent operator in 'SET ON/OFF' within a function."

What does the error message mean? How could I correct it? Thanks for answering.