【原创】Asp.net MVC 学习笔记之-为什么使用Html.TextBox方法会出现异常？
在上篇文章：【原创】Asp.net MVC学习笔记之-基于类型来绑定Model的属性 中，例举了如何使用基于类型和基于用途2种方式结合来限制Model的绑定，由此引发了一个意想不到的问题。
先看一下前台页面的代码：
代码
<
p
>
<
label
for
=
"
Test1
"
>
Test1:
</
label
>
<%=
Html.TextBox(
"
Test1
"
)
%>
<%=
Html.ValidationMessage(
"
Test1
"
,
"
*
"
)
%>
</
p
>
<
p
>
<
label
for
=
"
Test2
"
>
Test2:
</
label
>
<%=
Html.TextBox(
"
Test2
"
)
%>
<%=
Html.ValidationMessage(
"
Test2
"
,
"
*
"
)
%>
</
p
>
<
p
>
<
label
for
=
"
Test3
"
>
Test3:
</
label
>
<%=
Html.TextBox(
"
Test3
"
)
%>
<%=
Html.ValidationMessage(
"
Test3
"
,
"
*
"
)
%>
</
p
>
<
p
>
<
label
for
=
"
Test4
"
>
Test4:
</
label
>
<%=
Html.TextBox(
"
Test4
"
)
%>
<%=
Html.ValidationMessage(
"
Test4
"
,
"
*
"
)
%>
</
p
>
<
p
>
<
label
for
=
"
Test5
"
>
Test5:
</
label
>
<%=
Html.TextBox(
"
Test5
"
)
%>
<%=
Html.ValidationMessage(
"
Test5
"
,
"
*
"
)
%>
</
p
>
当页面输入的数据在Controller里经过数据绑定之后，只剩下了Test3一项有值，其他几个都是null,然后当程序走到
Html.TextBox的时候就出错了，报错如下：
代码
堆栈跟踪:
[NullReferenceException: 未将对象引用设置到对象的实例。]
System.Web.Mvc.HtmlHelper.GetModelStateValue(String key, Type destinationType) +63
System.Web.Mvc.Html.InputExtensions.InputHelper(HtmlHelper htmlHelper, InputType inputType, String name, Object value, Boolean useViewData, Boolean isChecked, Boolean setId, Boolean isExplicitValue, IDictionary`2 htmlAttributes) +519
System.Web.Mvc.Html.InputExtensions.TextBox(HtmlHelper htmlHelper, String name, Object value, IDictionary`2 htmlAttributes) +34
System.Web.Mvc.Html.InputExtensions.TextBox(HtmlHelper htmlHelper, String name) +60
ASP.views_dinners_create_aspx.__RenderContent2(HtmlTextWriter __w, Control parameterContainer) in e:\学习资料\我的学习文件\工作学习\【12】-网页方向\MVC\一步一步学asp.net_mvc\Projects\NerdDinner\NerdDinner\Views\Dinners\Create.aspx:30
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
System.Web.UI.Control.Render(HtmlTextWriter writer) +10
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
ASP.views_shared_site_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in e:\学习资料\我的学习文件\工作学习\【12】-网页方向\MVC\一步一步学asp.net_mvc\Projects\NerdDinner\NerdDinner\Views\Shared\Site.Master:26
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +256
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
System.Web.UI.Control.Render(HtmlTextWriter writer) +10
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +134
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +19
System.Web.UI.Page.Render(HtmlTextWriter writer) +29
System.Web.Mvc.ViewPage.Render(HtmlTextWriter writer) +59
System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +27
System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +99
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +25
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1266
找了半天也没找到原因，于是我把数据绑定的限制去掉试了一下，就没问题了，于是我想问题可能出在这个方法上面，祭出
Reflector,直接打开C盘下的System.Web.MVC.Dll，看个究竟：
通过HtmlHelper.TextBox方法，很容易跟到这个函数：
代码
private
static
string
InputHelper(
this
HtmlHelper htmlHelper, InputType inputType,
string
name,
object
value,
bool
useViewData,
bool
isChecked,
bool
setId,
bool
isExplicitValue, IDictionary
<
string
,
object
>
htmlAttributes)
{
ModelState state;
if
(
string
.IsNullOrEmpty(name))
{
throw
new
ArgumentException(MvcResources.Common_NullOrEmpty,
"
name
"
);
}
TagBuilder builder
=
new
TagBuilder(
"
input
"
);
builder.MergeAttributes
<
string
,
object
>
(htmlAttributes);
builder.MergeAttribute(
"
type
"
, HtmlHelper.GetInputTypeString(inputType));
builder.MergeAttribute(
"
name
"
, name,
true
);
string
str
=
Convert.ToString(value, CultureInfo.CurrentCulture);
bool
flag
=
false
;
switch
(inputType)
{
case
InputType.CheckBox:
{
bool
?
modelStateValue
=
htmlHelper.GetModelStateValue(name,
typeof
(
bool
))
as
bool
?
;
if
(modelStateValue.HasValue)
{
isChecked
=
modelStateValue.Value;
flag
=
true
;
}
break
;
}
case
InputType.Password:
if
(value
!=
null
)
{
builder.MergeAttribute(
"
value
"
, str, isExplicitValue);
}
goto
Label_0152;
case
InputType.Radio:
break
;
default
:
{
string
str3
=
(
string
) htmlHelper.GetModelStateValue(name,
typeof
(
string
));
builder.MergeAttribute(
"
value
"
, str3
??
(useViewData
?
htmlHelper.EvalString(name) : str), isExplicitValue);
goto
Label_0152;
}
}
if
(
!
flag)
{
string
a
=
htmlHelper.GetModelStateValue(name,
typeof
(
string
))
as
string
;
if
(a
!=
null
)
{
isChecked
=
string
.Equals(a, str, StringComparison.Ordinal);
flag
=
true
;
}
}
if
(
!
flag
&&
useViewData)
{
isChecked
=
htmlHelper.EvalBoolean(name);
}
if
(isChecked)
{
builder.MergeAttribute(
"
checked
"
,
"
checked
"
);
}
builder.MergeAttribute(
"
value
"
, str, isExplicitValue);
Label_0152:
if
(setId)
{
builder.GenerateId(name);
}
if
(htmlHelper.ViewData.ModelState.TryGetValue(name,
out
state)
&&
(state.Errors.Count
>
0
))
{
builder.AddCssClass(HtmlHelper.ValidationInputCssClassName);
}
if
(inputType
==
InputType.CheckBox)
{
StringBuilder builder2
=
new
StringBuilder();
builder2.Append(builder.ToString(TagRenderMode.SelfClosing));
TagBuilder builder3
=
new
TagBuilder(
"
input
"
);
builder3.MergeAttribute(
"
type
"
, HtmlHelper.GetInputTypeString(InputType.Hidden));
builder3.MergeAttribute(
"
name
"
, name);
builder3.MergeAttribute(
"
value
"
,
"
false
"
);
builder2.Append(builder3.ToString(TagRenderMode.SelfClosing));
return
builder2.ToString();
}
return
builder.ToString(TagRenderMode.SelfClosing);
}
这里注意Switch应该是走到default里，继续跟，发现这个函数有问题：
代码
internal
object
GetModelStateValue(
string
key, Type destinationType)
{
ModelState state;
if
(
this
.ViewData.ModelState.TryGetValue(key,
out
state))
{
return
state.Value.ConvertTo(destinationType,
null
);
}
return
null
;
}
其中这句话怎么看怎么不爽：return state.Value.ConvertTo(destinationType, null);
于是我在Controller里自己试了一把：
代码
System.Web.Mvc.ModelState state;
if
(
this
.ViewData.ModelState.TryGetValue(
"
Test1
"
,
out
state))
{
var result
=
state.Value.ConvertTo(
typeof
(
string
),
null
);
}
监视器打开，问题找到了
至于这后面隐藏了MVC的什么内部实现，等我搞清楚了再回头看看。今天的问题算是告一段落了。