如何在 ASP.NET CORE 1.1 中将视图呈现为字符串

Question

我想将部分视图呈现为字符串，我搜索并找到了这篇文章：

https://ppolyzos.com/2016/09/09/asp-net-core-render-view-to-string

public async Task<string> RenderToStringAsync(string viewName, object model)
{
    var httpContext = new DefaultHttpContext { RequestServices = _serviceProvider };
    var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());

    using (var sw = new StringWriter())
    {
        var viewResult = _razorViewEngine.FindView(actionContext, viewName, false);

        if (viewResult.View == null)
        {
            throw new ArgumentNullException($"{viewName} does not match any available view");
        }

        var viewDictionary = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary())
        {
            Model = model
        };

        var viewContext = new ViewContext(
            actionContext,
            viewResult.View,
            viewDictionary,
            new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
            sw,
            new HtmlHelperOptions()
        );

        await viewResult.View.RenderAsync(viewContext);

        return sw.ToString();
    }
}

但内部错误发生：

消息： Cannot convert null to 'bool' because it is a non-nullable value type

StackTrace: at CallSite.Target(Closure , CallSite , Object )\\r\\n at CallSite.Target(Closure , CallSite , Object )\\r\\n at AspNetCore._Views_Shared_Comments_cshtml.<ExecuteAsync>d__39.MoveNext() in /Views/Shared/Comments.cshtml:line 7\\r\\n--- End of stack trace fr...

我的错误是在innerException中，我无法确切知道原因是什么，但我认为在那行代码中：

var viewContext = new ViewContext(
    actionContext,
    viewResult.View,
    viewDictionary,
    new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
    sw,
    new HtmlHelperOptions()
);

我检查视图中的第 7 行，发现一些变量没有值并修复它，但出现另一个错误：

消息： Index was out of range. Must be non-negative and less than the size of the collection.\\r\\nParameter name: index Index was out of range. Must be non-negative and less than the size of the collection.\\r\\nParameter name: index

StackTrace: at System.ThrowHelper.ThrowArgumentOutOfRange_IndexException()\\r\\n at System.Collections.Generic.List``1.get_Item(Int32 index)\\r\\n at Microsoft.AspNetCore.Mvc.Routing.UrlHelper.GetVirtualPathData(String routeName, RouteValueDictionary values)\\r\\n a...

Answer 1

我无法重现确切的错误，但我有 2 个建议：

1.首先检查viewResult是否为NULL

viewResult可能是NULL因此 viewResult.View 可能会抛出NullReferenceException 。 所以我会这样做：

if (viewResult == null)
{
    throw new ArgumentNullException($"{ viewname } is not found.");
}

2. await View.RenderAsync()

var viewContext = new ViewContext(actionContext,
    viewResult.View,
    viewDictionary,
    new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
    sw,
    new HtmlHelperOptions()
);

await viewResult.View.RenderAsync(viewContext);

return sw.ToString();

Answer 2

我有同样的问题。 仔细观察后，我意识到，我在视图中使用了一个没有设置值的 ViewData。 因此出现错误“无法将 null 转换为 'bool'，因为它是不可为 null 的值类型”。 检查 Null 的值，为我解决了问题并且正确生成了视图。

Answer 3

您需要在/Views/Shared/Comments.cshtml:line 7处检查Viewbag/Tempdata 。

你没有从控制器发送值到视图，在这种情况下它是空的，所以为什么它会抛出运行时错误。