这是由于在使用Keycloak作为身份验证提供程序时，使用的Owin中间件和ASP.NET的集成方式可能不同导致的。解决此问题的步骤如下：

1.添加Owin中间件

在Global.asax.cs的Application_Start方法中，添加以下代码：

using Owin; using Keycloak.Owin.Authentication;

protected void Application_Start() { // ... other code ...

// Add Keycloak middleware to OWIN pipeline
app.UseKeycloakAuthentication(new KeycloakAuthenticationOptions
{
    Realm = "your_realm",
    ClientId = "your_client_id",
    ClientSecret = "your_client_secret",
    KeycloakUrl = "http://localhost:8080/auth/",
    SignInAsAuthenticationType = "Cookies"
});

}

2.配置Auth0 OWIN调用中间件

如果仍然存在IsAuthenticated属性始终为false的问题，那么可以尝试使用Auth0 OWIN调用中间件， 添加以下代码：

using Auth0.Owin; using Auth0.Owin.Authentication;

protected void Application_Start() { // ... other code ...

// Add Auth0 Owin middleware to OWIN pipeline
app.UseAuth0Authentication(new Auth0AuthenticationOptions
{
    ClientId = "your_client_id",
    ClientSecret = "your_client_secret",
    Domain = "your_domain.auth0.com",
    AuthenticationType = "Auth0",
    RedirectUri = "http://localhost:port/",
    PostLogoutRedirectUri = "http://localhost:port/",
    Scope = "openid email roles",
    TokenValidationParameters = new TokenValidationParameters
    {
        NameClaimType = "name",
        RoleClaimType = "http://schemas.microsoft.com/ws/2008/06/identity/claims/role"
    }
});

}

请注意，必须安装适当的软件包（例如Microsoft.Owin.Security.Keycloak和Auth0.Owin.Authentication），并导入适当的名称空间才能使这两种中间件正常工作。