SetDefaultValues

ISubtypes subtypes = (ISubtypes) targetFC;  // Get subtypes from the target feature class
IFeature newFeature = targetFC.CreateFeature();
if (subtypes.HasSubtype)
{
    // Assign defaults for the subtype
        IRowSubtypes rowSubtypes = (IRowSubtypes)newFeature;
        rowSubtypes.SubtypeCode = subtypes.DefaultSubtypeCode;
        rowSubtypes.InitDefaultValues();
        newFeature.Store();
}
else
{
        // Assign defaults for the feature
        IRowSubtypes rowSubtypes = (IRowSubtypes)newFeature;
        rowSubtypes.InitDefaultValues();
        newFeature.Store();
}