Dapper .NET: problems with struct parameterized queries

Question

I am trying to use structs as parameters in Dapper queries. It does not work for me. But it seems like it should work .

A failing example:

struct Simple { public int ID; }

[TestMethod]
public void StructParameter()
{
    int result = Db.Query<int>("select [ID] = @ID", new Simple { ID = 123 }).First();
    result.Should().Be(123);
}

A working example:

[TestMethod]
public void AnonymousParameter()
{
    int result = Db.Query<int>("select [ID] = @ID", new { ID = 123 }).First();
    result.Should().Be(123);
}

The error thrown in the first case:

System.Data.SqlClient.SqlException: Must declare the scalar variable "@ID".
Result StackTrace:  
at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action`1 wrapCloseInAction)
   at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose)
   at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady)
   at System.Data.SqlClient.SqlDataReader.TryConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async, Int32 timeout, Task& task, Boolean asyncWrite, SqlDataReader ds)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, TaskCompletionSource`1 completion, Int32 timeout, Task& task, Boolean asyncWrite)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
   at System.Data.Common.DbCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
   at Dapper.SqlMapper.<QueryImpl>d__11`1.MoveNext() in d:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 1553
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Dapper.SqlMapper.Query[T](IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Boolean buffered, Nullable`1 commandTimeout, Nullable`1 commandType) in d:\Dev\dapper-dot-net\Dapper NET40\SqlMapper.cs:line 1443
   ...

I am using Dapper version 1.38.

---

Per suggestion by Dirk, property instead of a filed works just fine:

struct Simple {
    public int ID { get; set; }
}

Answer 1

The problem is not caused by using structs, it's caused by using fields instead of properties.

If you'd change struct Simple into class Simple is still wouldn't work because the parameter reader creates only IL code to read properties.

Changing the ID field into a property will solve it.