Sample CustomPolicyProvider throws "Nullability of reference types" warning on .net 6

Question

I am trying to implement custom policy provider on .net 6(c#10). Due to some changes on c#10 compiler throws some warning for the following sample implmentation. Please provide a different implementation to remove the warnings.

https://github.com/dotnet/aspnetcore/blob/v3.1.3/src/Security/samples/CustomPolicyProvider/Authorization/MinimumAgePolicyProvider.cs

Answer 1

Hi @M.H. Mukit ,

You can F12 to step to the source code of IAuthorizationPolicyProvider in Visual Studio and you can see the two method of the response data is nullable. You can add ? to allow nullable. Here is a whole working sample you could follow:

internal class MinimumAgePolicyProvider : IAuthorizationPolicyProvider  
{  
    const string POLICY_PREFIX = "MinimumAge";  
    public DefaultAuthorizationPolicyProvider FallbackPolicyProvider { get; }  
  
    public MinimumAgePolicyProvider(IOptions<AuthorizationOptions> options)  
    {  
        // ASP.NET Core only uses one authorization policy provider, so if the custom implementation  
        // doesn't handle all policies (including default policies, etc.) it should fall back to an  
        // alternate provider.  
        //  
        // In this sample, a default authorization policy provider (constructed with options from the   
        // dependency injection container) is used if this custom provider isn't able to handle a given  
        // policy name.  
        //  
        // If a custom policy provider is able to handle all expected policy names then, of course, this  
        // fallback pattern is unnecessary.  
        FallbackPolicyProvider = new DefaultAuthorizationPolicyProvider(options);  
    }  
  
    public Task<AuthorizationPolicy> GetDefaultPolicyAsync() => FallbackPolicyProvider.GetDefaultPolicyAsync();  
  
    public Task<AuthorizationPolicy?> GetFallbackPolicyAsync() => FallbackPolicyProvider.GetFallbackPolicyAsync();  
  
    // Policies are looked up by string name, so expect 'parameters' (like age)  
    // to be embedded in the policy names. This is abstracted away from developers  
    // by the more strongly-typed attributes derived from AuthorizeAttribute  
    // (like [MinimumAgeAuthorize] in this sample)  
    public Task<AuthorizationPolicy?> GetPolicyAsync(string policyName)  
    {  
        if (policyName.StartsWith(POLICY_PREFIX, StringComparison.OrdinalIgnoreCase) &&  
            int.TryParse(policyName.Substring(POLICY_PREFIX.Length), out var age))  
        {  
            var policy = new AuthorizationPolicyBuilder();  
            policy.AddRequirements(new MinimumAgeRequirement(age));  
            return Task.FromResult(policy?.Build());  
        }  
  
        // If the policy name doesn't match the format expected by this policy provider,  
        // try the fallback provider. If no fallback provider is used, this would return   
        // Task.FromResult<AuthorizationPolicy>(null) instead.  
        return FallbackPolicyProvider.GetPolicyAsync(policyName);  
    }  
}  

Here is also another way which can keep the previous code, you can remove the <Nullable>enable</Nullable> in your project file.

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Best Regards,
Rena

Answer 2

Hi @Rena Ni - MSFT

Thank you for the suggestion. I did some mistakes previously. I used your advice accordingly, and it removed the warning from occurring.. Thanks again.