返回类型为“字符串"的函数将返回可为空的字符串(即“字符串?")

对于Azure Functions项目，我正在使用具有启用的可空引用类型的C#8.由于使用AzureFunctions AppSettings是通过环境变量公开的，环境变量可以是字符串或null(即它们返回 string?)，因此我试图封装获取环境变量的逻辑-如果它们是未设置-设置为单独的方法 GetEnvVariable .

For an Azure Functions project I am using C# 8 with enabled nullable reference types. Since with AzureFunctions AppSettings are exposed via environment variables, which can be a string or null (i.e. they return string?) I tried to encapsulate the logic of getting environment variables - and throwing an error if they're not set - into a separate method GetEnvVariable.

如您所见， GetEnvVariable 通过三行简短的代码实现.如果未设置环境变量(即为 null )，则会引发自定义异常，否则将字符串值返回.

As you can see, GetEnvVariable is implemented in three short lines. If the environment variable is not set (i.e it is null) it throws a custom exception, else the string value is returned.

以下行未显示可能取消引用null"的问题，因为编译器知道，在此行 env 必须为 string (不是 string?)，因为该异常将已经触发并且无法到达此行.

The following line does not show a problem of "possible de-reference of null" because the compiler know, that at this line env must be string (not string?) because the exception would have fired already and this line could not have been reached.

var length = env.Contains('$');

var length = env.Contains('$');

仍然，当用鼠标悬停在变量 env 上时，类型显示为 string?

Still, when hovering over the variable env with the mouse the type is displayed as string?

这有意义吗?如果是这样，为什么?为 env 分配了 GetEnvVariable 方法的输出，该方法返回 string .

Does this make sense? If so, why? env is assigned the output of the GetEnvVariable method, which return string.

引发异常是否隐式返回 null ?请帮助我理解，非常感谢您的投入.干杯！

Does throwing an exception implicitly return null? Please help me understand, your input is really appreciated. Cheers!

这是我的代码

namespace delme_azf
{
    public static class kv
    {
        [FunctionName("kv")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = null)] HttpRequest req,
            ILogger log)
        {
            try
            {
                var envName = "lolomat";
                var env = GetEnvVariable(envName);
                var length = env.Contains('$');
                return new OkObjectResult($"success {env}");
            }
            catch (System.Exception)
            {
                throw;
            }
        }


        public static string GetEnvVariable(string name)
        {
            var env = Environment.GetEnvironmentVariable(name);
            if (env == null) throw new EnvironmentVariableNotSetException(nameof(name));
            return env;
        }


        public class EnvironmentVariableNotSetException : Exception
        {
            public EnvironmentVariableNotSetException() {}
            public EnvironmentVariableNotSetException(string envName) : base($"Mandatory environment variable '{envName}' is not set.") {}
        }
    }
}

这也是我完整的* .csproj，显示了可空引用类型已全局启用.

Also here's my full *.csproj showing that nullable reference types are enabled globally.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <AzureFunctionsVersion>v3</AzureFunctionsVersion>
    <RootNamespace>delme_azf</RootNamespace>
  </PropertyGroup>
  <PropertyGroup>
    <Nullable>enable</Nullable>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.Azure.Functions.Extensions" Version="1.0.0" />
    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="3.0.3" />
    <PackageReference Include="Oracle.ManagedDataAccess.Core" Version="2.19.70" />
  </ItemGroup>
  <ItemGroup>
    <None Update="host.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Update="local.settings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <CopyToPublishDirectory>Never</CopyToPublishDirectory>
    </None>
  </ItemGroup>
</Project>