HtmlHelper扩展ModelState取不出值

代码如下，

 protected internal string GetFieldNameTValue(ExpressionFuncTModel, TValue expression)
        {
            ModelMetadata modelMetadataByExpression = ModelMetadata.FromLambdaExpressionTModel, TValue(expression, ((HtmlHelperTModel)HtmlHelperExtension.HtmlHelper).ViewData);
            string expressionText = ExpressionHelper.GetExpressionText(expression);
            return HtmlHelperExtension.HtmlHelper.ViewData.TemplateInfo.GetFullHtmlFieldName(expressionText);
        }
                 protected internal dynamic GetModelStateValueTValue(ExpressionFuncTModel, TValue expression)
        {
            string key = this.GetFieldName(expression);
            ModelMetadata modelMetadata = ModelMetadata.FromLambdaExpressionTModel, TValue(expression, ((HtmlHelperTModel)HtmlHelperExtension.HtmlHelper).ViewData);
            ModelState modelState;
            if (HtmlHelperExtension.HtmlHelper.ViewData.ModelState.TryGetValue(key, out modelState) && modelState.Value != null)
            {
                return modelState.Value.ConvertTo(modelMetadata.Model.GetType(), null);
            }
            else
                return null;
        }

取出的modelState永远是null ，是不是漏了什么？

参考：http://stackoverflow.com/questions/7735179/html-helper-to-get-values-from-model-state