实现IDbConnection.ConnectionString属性时遭遇CS8767警告的问询

当实现.NET 8的IDbConnection接口时，ConnectionString属性的setter出现CS8767警告，核心原因是接口定义中setter参数允许接受null值，但你的隐式实现默认要求setter的value参数为非空引用类型，二者可空性不匹配。

你尝试使用[param: AllowNull]标注参数的方式无效，是因为对于属性setter的可空性调整，正确的做法是直接在setter上标注[AllowNull]特性，而非参数级别标注。另外也可以通过显式接口实现来自动匹配接口的可空规则，以下是两种可行的解决方案：

方案一：使用[AllowNull]标注setter #

需要引入System.Diagnostics.CodeAnalysis命名空间，直接在setter上添加特性：

using System.Data.Common;
using System.Diagnostics.CodeAnalysis;

class Foo : IDbConnection
{
    public string ConnectionString
    {
        get => throw new NotImplementedException();
        [AllowNull]
        set => throw new NotImplementedException();
    }

    // 实现其他接口成员
    public int ConnectionTimeout => throw new NotImplementedException();
    public string Database => throw new NotImplementedException();
    public ConnectionState State => throw new NotImplementedException();
    public IDbTransaction BeginTransaction() => throw new NotImplementedException();
    public IDbTransaction BeginTransaction(IsolationLevel il) => throw new NotImplementedException();
    public void ChangeDatabase(string databaseName) => throw new NotImplementedException();
    public void Close() => throw new NotImplementedException();
    public IDbCommand CreateCommand() => throw new NotImplementedException();
    public void Open() => throw new NotImplementedException();
    public void Dispose() => throw new NotImplementedException();
}

方案二：显式实现IDbConnection接口 #

显式实现时，编译器会自动继承接口的可空性定义，无需手动添加特性，直接匹配接口要求：

using System.Data.Common;

class Foo : IDbConnection
{
    string IDbConnection.ConnectionString
    {
        get => throw new NotImplementedException();
        set => throw new NotImplementedException();
    }

    // 实现其他接口成员
    public int ConnectionTimeout => throw new NotImplementedException();
    public string Database => throw new NotImplementedException();
    public ConnectionState State => throw new NotImplementedException();
    public IDbTransaction BeginTransaction() => throw new NotImplementedException();
    public IDbTransaction BeginTransaction(IsolationLevel il) => throw new NotImplementedException();
    public void ChangeDatabase(string databaseName) => throw new NotImplementedException();
    public void Close() => throw new NotImplementedException();
    public IDbCommand CreateCommand() => throw new NotImplementedException();
    public void Open() => throw new NotImplementedException();
    public void Dispose() => throw new NotImplementedException();
}

两种方案都能消除CS8767警告，你可以根据代码架构需求选择合适的方式。

内容的提问来源于stack exchange，提问作者RobSiklos