What is wrong with this CIL?

Question

I'm generating some code via Cecil. The code generates without error, but when I try to load the assembly I get:

An unhandled exception of type 'System.InvalidProgramException' occurred in DataSerializersTest.exe

Additional information: Common Language Runtime detected an invalid program.

Here is the generated IL:

.method public static 
    class Data.FooData Read (
        class [mscorlib]System.IO.BinaryReader input
    ) cil managed 
{
    // Method begins at RVA 0x3a58
    // Code size 60 (0x3c)
    .maxstack 3
    .locals (
        [0] class Data.FooData,
        [1] valuetype [System.Runtime]System.Nullable`1<int32>
    )

    IL_0000: newobj instance void Data.FooData::.ctor()
    IL_0005: stloc.0
    IL_0006: ldloc.0
    IL_0007: ldarg.0
    IL_0008: callvirt instance bool [System.IO]System.IO.BinaryReader::ReadBoolean()
    IL_000d: ldc.i4.0
    IL_000e: ceq
    IL_0010: brtrue.s IL_001f

    IL_0012: ldarg.0
    IL_0013: callvirt instance int32 [System.IO]System.IO.BinaryReader::ReadInt32()
    IL_0018: newobj instance void valuetype [System.Runtime]System.Nullable`1<int32>::.ctor(!0)
    IL_001d: br.s IL_0028

    IL_001f: nop
    IL_0020: ldloca.s 1
    IL_0022: initobj valuetype [System.Runtime]System.Nullable`1<int32>

    IL_0028: nop
    IL_0029: callvirt instance void Data.FooData::set_I(valuetype [System.Runtime]System.Nullable`1<int32>)
    IL_002e: ldloc.0
    IL_002f: ldarg.0
    IL_0030: callvirt instance string [System.IO]System.IO.BinaryReader::ReadString()
    IL_0035: callvirt instance void Data.FooData::set_Name(string)
    IL_003a: ldloc.0
    IL_003b: ret
} // end of method FooDataReader::Read

The equivalent C# code looks like this:

public static FooData Read(BinaryReader input)
{
    var result = new FooData();

    if (input.ReadBoolean())
    {
        result.I = input.ReadInt32();
    }
    else
    {
        result.I = null;
    }

    result.Name = input.ReadString();

    return result;
}

I've worked through the IL several times and it makes sense to me. It's not the same as what the C# compiler produces for the above C# code, but I'd like to understand what's wrong with the IL I've generated. Can anyone tell me?

Answer 1

I think I've spotted it. initobj does not leave the newly initialized object on the evaluation stack - whereas newobj does. So the stacks are unbalanced when IL_0028 is reached - if we've gone the newobj route, then we have two items on the stack (FooData and Nullable<int>). If we've gone the initobj route, then all we have is the FooData object reference on the stack. You need to reload local 1:

IL_0022: initobj valuetype [System.Runtime]System.Nullable`1<int32>
         ldloc.1
IL_0028: nop