成员表达式不能转换为空的从十进制到对象 [英] Member Expression cannot convert to object from nullable decimal
问题描述
我工作的一个MVC项目,想通过Html.TextboxFor方法的属性的名称。这是我的视图模型
I am working on an MVC project and would like to pass the Html.TextboxFor method the name of a property. This is my viewmodel
public class RuleViewModel<T> where T : class, IValidatableObject
{
private T _rule;
public T RuleModel
{
get
{
return _rule
?? (_rule = Activator.CreateInstance<T>());
}
}
public RuleMetadata Metadata { get; set; }
public Expression<Func<RuleViewModel<T>, Object>> GetParameterByName(PropertyInfo pi)
{
var fieldName = Expression.Parameter(typeof(RuleViewModel<T>), "x");
var fieldExpression = Expression.PropertyOrField(Expression.Property(fieldName, "RuleModel"), pi.Name);
var exp = Expression.Lambda<Func<RuleViewModel<T>, Object>>(fieldExpression, fieldName);
return exp;
}
}
然后在视图我这样做
Then in the view I do this
@foreach (var prop in Model.RuleModel.GetType().GetProperties())
{
var result = Model.Metadata.Columns.SingleOrDefault(m => m.ColumnName == prop.Name);
if (result != null)
{
<td>
@Html.TextBoxFor(Model.GetParameterByName(prop))
</td>
}
}
现在的问题是,当属性的类型为十进制?,我得到一个不能转换为空的小数对象错误。我环顾四周,发现可以使用Expression.Convert来解决这个问题,但是当我这样做,我的观点得到一个错误
The problem is that when the property is of type decimal?, I get a cannot convert nullable decimal to object error. I looked around and found that you can use Expression.Convert to fix this but when I do that I get an error on the view
模板只能用于字段访问使用,属性访问,一维数组的索引,或单参数自定义索引表达式。
Templates can be used only with field access, property access, single-dimension array index, or single-parameter custom indexer expressions.
任何帮助,将不胜感激。这是我的工作,没有这一块是一潭死水概念项目的证明。
Any help would be appreciated. This is a proof of concept project that I'm working on and without this piece it is dead in the water.
推荐答案
的问题是,你不能只使用对象
为 TProperty
打电话时 TextBoxFor<的TModel, TProperty>()
。该公司预计形式的lambda表达式 Func键<的TModel,TProperty>
和C#的变化规则是这样的: Func键<的TModel,小数? >
不是分配与 Func键LT兼容&;的TModel,对象>
。你也不能简单地用转换()
,因为MVC内部不会接受一个lambda,其主体是转换$ C $ 。C>表达式
The problem is that you can't just use object
as TProperty
when calling TextBoxFor<TModel, TProperty>()
. It expects a lambda expression of the form Func<TModel, TProperty>
, and the variance rules of C# are such that a Func<TModel, decimal?>
is not assignment compatible with Func<TModel, object>
. You also can't simply use Convert()
, because the MVC internals won't accept a lambda whose body is a Convert
expression.
在<什么EM> 的可以做的是使用动态绑定来调用 TextBoxFor<的TModel,TProperty>()
正确类型的参数:
What you can do is use dynamic binding to invoke TextBoxFor<TModel, TProperty>()
with the correct type arguments:
public Expression GetParameterByName(PropertyInfo pi)
{
var fieldName = Expression.Parameter(typeof(RuleViewModel<T>), "x");
var fieldExpression = Expression.PropertyOrField(
Expression.Property(fieldName, "RuleModel"),
pi.Name);
var exp = Expression.Lambda(
typeof(Func<,>).MakeGenericType(typeof(RuleViewModel<T>), fieldExpression.Type),
fieldExpression,
fieldName);
return exp;
}
// ...
@InputExtensions.TextBoxFor(Html, (dynamic)Model.GetParameterByName(prop))
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