c# - System.ArgumentNullException : Value cannot be null, 参数名称: implementationInstance

Question

我在 IIS 中部署了 .NET core mvc 应用程序，当我运行应用程序时，页面显示 502.5 错误，我在 powershell "dotnet D:\deploy\WebApp\WebApp.dll"中运行命令，下面显示详细的错误内容:

注意: .net 核心版本 2.0.0

Unhandled Exception: System.ArgumentNullException: Value cannot be null.

Parameter name: implementationInstance

at Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions
.AddSingleton[TService](IServiceCollection services, TService implementationInstance)

I know how the error occurred, how to instantiate?

public class Startup
{   ...
    public void ConfigureServices(IServiceCollection services)
    {
        ...
        services.AddSingleton(CreateQuery());  // this is error location
        ...
    }

    IQuery CreateQuery()
    {
        IQuery query = null;
        var dataBase = Configuration.GetSection("AppSettings")["DataBase"];
        var defaultConnection = Configuration.GetSection("ConnectionStrings")["SqlServer"];
        switch (dataBase)
        {
            case "sqlserver":
                query = new WebApp.Query.Query(defaultConnection);
                break;
        }
        return query;
    }
}

Answer 1

如果有人来寻找可能的解决方案，我可以通过使用 IServiceProvider 获取配置来解决 .NET Core 3.1 项目中的这个问题。回调由 .AddSingleton 提供通过以下方式:

public class Startup
{   ...
    public void ConfigureServices(IServiceCollection services)
    {
        ...
        // Pass the service provider that AddSingleton provides
        services.AddSingleton((serviceProvider) => {
            CreateQuery(serviceProvider)
        });
        ...
    }

    IQuery CreateQuery(IServiceProvider serviceProvider)
    {
        IQuery query = null;

        // Use the service provider to get the configuration
        var configuration = serviceProvider.GetRequiredService<IConfiguration>();

        var dataBase = configuration.GetSection("AppSettings")["DataBase"];
        var defaultConnection = configuration.GetSection("ConnectionStrings")["SqlServer"];
        switch (dataBase)
        {
            case "sqlserver":
                query = new WebApp.Query.Query(defaultConnection);
                break;
        }
        return query;
    }
}