Have a look at this page on the API help file, whilst it's about creating subtypes you should be able to figure out how to access existing subtypes.
public void GetSubtypes(IFeatureClass featureClass) { // Cast the feature class to the ISubtypes interface. ISubtypes subtypes = (ISubtypes)featureClass; // then you can enumeraate subtypes. IEnumSubtype enumSubtype = subtypes.Subtypes
}
You Can Use the Following Code (C#) -
//here item is a IfeatureClass
ISubtypes subtypes = item as ISubtypes;
if (subtypes.HasSubtype)
{
IEnumSubtype enumSubtype = subtypes.Subtypes;
enumSubtype.Reset();
int subcode = -1;
string subdesc = enumSubtype.Next(out subcode);
while (!string.IsNullOrEmpty(subdesc))
{
subdesc = enumSubtype.Next(out subcode);
//str3 is a string (empty)
str3 = subdesc;
}
}
//in this above code you can able to find subtype names associated with each featurelayer