How to get method parameter's signature?

Hi, I have to save  method parameter's signature to do sth.

I can parser the signature from IMetaDataImport::GetMethodProps and get method parameter's signature rightly, but it's dirty because I have to split parameters and save each parameter's beginning and ending. So I try another way below, but it's not work.

I use IMetaDataImport::GetParamForMethodIndex to do get all parameters' mdParamDef, then use IMetaDataImport::GetSigFromToken with mdParamDef to get signature.

IMetaDataImport::GetSigFromToken returns 0x80131124 for some mdParamDef, and what's the meaning?

I parser the signature return by IMetaDataImport::GetSigFromToken. The result of parameter's type all are ELEMENT_TYPE_U2. Obviously, it is not right. 

Can you tell me why? Thanks a lot!