System.ArgumentNullException:值不能为null，参数名称:ImplementationInstance [英] System.ArgumentNullException: Value cannot be null, Parameter name: implementationInstance

问题描述

我在IIS中部署了.NET核心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，以下显示详细的错误内容:

I deployed .NET core mvc application in IIS, when I run app, the page show 502.5 error, I run command in powershell "dotnet D:\deploy\WebApp\WebApp.dll" ,this follow show detail error content:

注意: .net核心版本2.0.0

NOTE:.net core version 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)

我知道错误是如何发生的，如何实例化?

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;
    }
}

推荐答案

如果有人来寻找可能的解决方案，我可以通过使用.AddSingleton通过以下方式提供的>回调:

In case someone comes along looking for a possible solution, I was able to resolve this issue in a .NET Core 3.1 project by getting the configuration using the IServiceProvider callback provided by .AddSingleton in the following way:

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;
    }
}

这篇关于System.ArgumentNullException:值不能为null，参数名称:ImplementationInstance的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！