How to add a reference from XmlAttributeValue to PsiClass？

I created a ReferenceContributor and then register some ReferenceProviders：

public class MyXmlReferenceContributor extends PsiReferenceContributor {

    @Override
    public void registerReferenceProviders(@NotNull PsiReferenceRegistrar registrar) {
        registrar.registerReferenceProvider(xmlAttributeValue(),
                new PsiReferenceProvider() {})

It seems to be all right，when it jump from the xmlAttributeValue to the declaration of psiClass.

But it couldn't  find Usages what i wanted in the PsiClass when ctrl+B .

By the way , I tried to use ：

ReferencesSearch.search(psiClass).findAll()

，the results were disappointing.

It puzzled me ！！！

If i edited the “demo_MyUserModel” to "demo.MyUserModel" or "demo$MyUserModel" or "demo/MyUserModel" ，it works for me.

Why？What should I do？What’s wrong with the ' _ ' ?