Nullable Reference Types and the Options Pattern

问题:

How can we use non-nullable reference types in combination with the Options pattern?

Let's say we have an options model named MyOptions.

The services requiring those options are getting IOptions<MyOptions> options injected into the constructor.

Configuring the options happens on the IServiceCollection like this:

services
    .AddOptions<MyOptions>()
    .Configure(options =>
    {
        options.Name = "ABC";
    });

Now, the problem is in the definition of MyOptions:

public sealed class MyOptions
{
    public string Name { get; set; }
}

which generates the warning:

CS8618 Non-nullable property 'Name' is uninitialized. Consider declaring the property as nullable.

1. We don't want to make Name nullable as then we need to place traditional null checks everywhere (which is against the purpose of non-nullable reference types)
2. We can't create a constructor to enforce the MyOptions class to be created with a non-nullable name value as the Configure method construct the options instance for us
3. We can't use the null-forgiving operator trick (public string name { get; set; } = null!;) as then we can't ensure the Name property is set and we can end up with a null in the Name property where this would not be expected (inside the services)

Any other option I forgot to consider?

回答1:

It seems, that you have two possible options here. First one is to initialize an Options properties using empty string (instead of null value) to avoid null checks

public sealed class MyOptions
{
    public string Name { get; set; } = "";
}

Second one is to make all of the properties a nullable ones and decorate them using DisallowNull precondition and NotNull postcondition.

DisallowNull means that nullable input argument should never be null, NotNull - a nullable return value will never be null. But these attributes only affect nullable analysis for the callers of members that are annotated with them. So, you are indicating that your property can never return null or be set to null, despite nullable declaration

public sealed class MyOptions
{
    [NotNull, DisallowNull]public string? Name { get; set; }
}

and the usage example

var options = new MyOptions();
options.Name = null; //warning CS8625: Cannot convert null literal to non-nullable reference type.
options.Name = "test";

But the next example doesn't show a warning, because nullable analysis doesn't work properly in object initializers yet, see GitHub issue 40127 in Roslyn repository.

var options = new MyOptions { Name = null }; //no warning

(Edit: This issue was fixed already, shipped in version 16.5 in March, 2020 and should go away after updating a VS to the latest version.)

The same picture for property getter, the following sample doesn't show any warnings, because you indicated that nullable return type can't be null

var options = new MyOptions();
string test = options.Name.ToLower();

but attempting to set a null value and get it generates a warning (compiler is smart enough to detect such scenarios)

var options = new MyOptions() { Name = null };
string test = options.Name.ToLower(); //warning CS8602: Dereference of a possibly null reference.

回答2:

If the expected behavior of the property is that it may initially contain null but should never be set to null, then try using DisallowNullAttribute.

#nullable enable

using System.Diagnostics.CodeAnalysis;

public sealed class MyOptions
{
    [DisallowNull]
    public string? Name { get; set; }

    public static void Test()
    {
        var options = new MyOptions();
        options.Name = null; // warning
        options.Name = "Hello"; // ok
    }

    public static void Test2()
    {
        var options = new MyOptions();
        options.Name.Substring(1); // warning on dereference
    }
}