如何为验证对象属性的方法正确使用nullable分析属性

问题原因 #

C# 可空引用类型的静态分析默认不会跨方法校验内部逻辑，你在Validate方法中实现的空检查规则无法被分析器感知，因此调用完Validate后，分析器仍认为可空属性可能为null。此外你现有Validate方法的三元判断逻辑写反了，当前逻辑为属性为空时不抛异常、非空时才抛出，需要先修正这个逻辑错误。

解决步骤 #

首先引入System.Diagnostics.CodeAnalysis命名空间，使用官方提供的可空分析特性告知分析器你的校验规则：

1. 标记Validate方法的非空承诺 #

给Validate方法添加[MemberNotNull]特性，声明该方法正常返回（无异常抛出）时，指定的属性一定不为null：

using System.Diagnostics.CodeAnalysis;

public class CosmosDbOptions
{
    public string? DatabaseName { get; set; }
    public string? InitialContainerName { get; set; }
#pragma warning disable CA1056 // URI-like properties should not be strings
    public string? EndpointUrl { get; set; }
#pragma warning restore CA1056 // URI-like properties should not be strings
    public string? AuthorizationKey { get; set; }
    public string? AzureADTenantId { get; set; }
    public TimeSpan? Timeout { get; set; }

    public AuthenticationMode AuthenticationMode => GetAuthenticationMode();
    public string? KeyVaultKeyName { get; private set; }

    private AuthenticationMode GetAuthenticationMode()
    {
        if (!string.IsNullOrWhiteSpace(AzureADTenantId))
        {
            return AuthenticationMode.AzureActiveDirectoryManagedIdentity;
        }
        else if (!string.IsNullOrWhiteSpace(KeyVaultKeyName))
        {
            return AuthenticationMode.AuhtorizationKeyFromKeyVault;
        }
        else if (!string.IsNullOrWhiteSpace(AuthorizationKey))
        {
            return AuthenticationMode.AuthorizationKey;
        }

        return AuthenticationMode.None;
    }

    [MemberNotNull(nameof(DatabaseName), nameof(InitialContainerName), nameof(EndpointUrl))]
    public void Validate()
    {
        // 修正三元判断逻辑，为空时抛异常
        DatabaseName = string.IsNullOrWhiteSpace(DatabaseName) ? throw new ArgumentNullException(nameof(DatabaseName)) : DatabaseName;
        InitialContainerName = string.IsNullOrWhiteSpace(InitialContainerName) ? throw new ArgumentNullException(nameof(InitialContainerName)) : InitialContainerName;
        EndpointUrl = string.IsNullOrWhiteSpace(EndpointUrl) ? throw new ArgumentNullException(nameof(EndpointUrl)) : EndpointUrl;

        if (AuthenticationMode == AuthenticationMode.None)
        {
            throw new NotSupportedException("Authentication mode for accessing CosmosDb is not valid!");
        }

        if (AuthenticationMode == AuthenticationMode.AuthorizationKey)
        {
            if (string.IsNullOrWhiteSpace(AuthorizationKey))
            {
                throw new ArgumentNullException(nameof(AuthorizationKey));
            }
        }

        if (AuthenticationMode == AuthenticationMode.AzureActiveDirectoryManagedIdentity)
        {
            if (string.IsNullOrWhiteSpace(AzureADTenantId))
            {
                throw new ArgumentNullException(nameof(AzureADTenantId));
            }
        }
    }

    // 新增分支判断辅助方法
    [MemberNotNullWhen(true, nameof(AuthorizationKey))]
    public bool UseAuthorizationKeyMode() => AuthenticationMode == AuthenticationMode.AuthorizationKey;

    [MemberNotNullWhen(true, nameof(AzureADTenantId))]
    public bool UseAzureAdMode() => AuthenticationMode == AuthenticationMode.AzureActiveDirectoryManagedIdentity;
}

public enum AuthenticationMode
{
    AuthorizationKey,
    AuhtorizationKeyFromKeyVault,
    AzureActiveDirectoryManagedIdentity,
    None
}

2. 调整调用侧判断逻辑 #

将分支判断改为使用新增的辅助方法，分析器就能识别对应分支下的属性非空：

public class Test
{
    public void DoSomeAction(CosmosDbOptions options)
    {
        _ = options ?? throw new ArgumentNullException(nameof(options));
        options.Validate();
        string x = options.DatabaseName; // 无空警告
        
        if (options.UseAuthorizationKeyMode())
        {
            string y = options.AuthorizationKey; // 无空警告
        }
    }
}

上述方案兼容.NET Core 3.0及以上所有版本，无需使用空包容运算符就能消除所有合法场景下的空警告。 #

内容的提问来源于stack exchange，提问作者Mahdi