Question

我在IIS中部署了.NET核心mvc应用程序，当我运行app时，页面显示502.5错误，我在powershell中运行命令＆＃34; dotnet D：\ deploy \ WebApp \ WebApp.dll＆＃34; ，以下显示详细错误内容：

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)

我知道错误是怎么发生的，如何实例化？

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

万一有人来寻找可能的解决方案，我可以通过使用以下IServiceProvider提供的.AddSingleton回调获取配置来解决.NET Core 3.1项目中的此问题。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;
    }
}

System.ArgumentNullException：Value不能为null，参数名称：implementationInstance

1 个答案: