Why this violates the constraint of type parameter 'TUser'?

Question

I am trying to customize asp.net identity entities with the following ones:

public class User : IdentityUser<string, UserClaim, UserRole, UserLogin>
public class UserClaim : IdentityUserClaim<string>
public class UserLogin : IdentityUserLogin<string>
public class UserRole : IdentityUserRole<string>
public class UserToken : IdentityUserToken<string>
public class Role : IdentityRole<string, UserRole, RoleClaim>
public class RoleClaim : IdentityRoleClaim<string>

I have then created a DbContext class like the following

public class AppDbContext : IdentityDbContext<User, Role, string, UserClaim, UserRole, UserLogin, RoleClaim, UserToken>

And then configure these with

services
    .AddIdentity<User, Role>()
    .AddEntityFrameworkStores<AppDbContext>()
    .AddDefaultTokenProviders();

but I have tried also with

.AddEntityFrameworkStores<AppDbContext, string>()

I have read many blog posts on the internet, like this for example, but most of them have to deal against a change of data type for the keys, like to int or Guid. In my case I am not changing the default key datatype from string.

I all these cases the compile is ok but it throws an error when running

System.TypeLoadException GenericArguments[0],'MyIdentity.Entities.User', on 'Microsoft.AspNetCore.Identity.EntityFrameworkCore.UserStore`8[TUser,TRole,TContext,TKey,TUserClaim,TUserRole,TUserLogin,TUserToken]' violates the constraint of type parameter 'TUser'.

at System.RuntimeTypeHandle.Instantiate(RuntimeTypeHandle handle, IntPtr* pInst, int numGenericArgs, ObjectHandleOnStack type) at System.RuntimeTypeHandle.Instantiate(Type[] inst) at System.RuntimeType.MakeGenericType(Type[] instantiation)

If I add a customized UserStore class like explained in this post

public class UserStore : UserStore<User, Role, AppDbContext, string, UserClaim, UserRole, UserLogin, UserToken>

A compile error appear saying that

CS0311 The type 'MyIdentity.Entities.Role' cannot be used as type parameter 'TRole' in the generic type or method 'UserStore'. There is no implicit reference conversion from 'MyIdentity.Entities.Role' to 'Microsoft.AspNetCore.Identity.EntityFrameworkCore.IdentityRole>'

What I am doing wrong?

Answer 1

• According to MSDN, the constraint for UserStore<TUser> is TUser : IdentityUser.
• Also according to MSDN, IdentityUser : IdentityUser<string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim>.
• According to your declarations, User : IdentityUser<string, UserClaim, UserRole, UserLogin>.

The long names make it easy to miss, so let's alias IdentityUser as D and IdentityUser<string, IdentityUserLogin, IdentityUserRole, IdentityUserClaim> as B. Now the above can be restated as:

• TUser : D (a TUser must be a kind of D)
• D : B (D is a kind of B)
• User : B (your User is a kind of B in general but not a kind of D specifically, as required)