blocks|key|1576387|text|我不清楚您的代码到底是什么，但是要获得非空版本的Nullable，请调用它的.Value成员。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1576388|entityMap^0|12|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

I'm not sure exactly what your code is, but to get the non-nullable version of a Nullable, call its <code>.Value</code> member.

blocks|key|1607256|text|无论您的比较是什么，请按以下方式更改比较：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1607257|(nullableDT+>=+DT)|code-block|syntax|javascript|1607258|至|1607259|(nullableDT+!=+null+&&+nullableDT.Value+>=+DT)|1607260|编辑：|1607261|根据您的注释，编写一个包含2个对象的函数，在函数中检查它们是否为空类型，并检查null，然后比较值。此函数可能使用类似于%5E的代码。|1607262|不过，这听起来好像你有一个更大的潜在问题。要么你得到了不正确的数据。其他地方的代码正在返回不应该是的数据，而不是代码中的问题，而是逻辑中的问题)，或者您认为可以比较这些对象的假设是无效的。这又一次是一个逻辑错误。|1607263|我想你应该退后一小会儿。如果你张贴更多的代码，我们可能会帮助你更多。|1607264|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|Y|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

where ever your compare is, change the compare like this:

<pre><code>(nullableDT &gt;= DT)
</code></pre>

To

<pre><code>(nullableDT != null &amp;&amp; nullableDT.Value &gt;= DT)
</code></pre>

Edit:

As per you comment, Write a function that takes 2 objects, inside the function check if they are nullable types, and check for null, then compare values. This function will probably use code similar to ^.

Though, this is starting to sound like you have a bigger underlying problem. Either you are getting incorrect data, (ie. your code elsewhere is returning data that it shouldn't be, not a problem in the code, but a problem in your logic,) or your assumption that you can compare these objects is invalid. Which, once again, is a logic error.

I think you might need to take a step back on this one for a minute. If you post more code, we might be able to help you some more.

blocks|key|1607316|text|很有趣，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1607317|我尝试过这两种代码的变体：|1607318|++++System.Nullable<DateTime>+now+=+new+System.Nullable<DateTime>();
++++now+=+DateTime.Now;|code-block|syntax|javascript|1607319|和|1607320|++++System.Nullable<DateTime>+now;
++++now+=+DateTime.Now;|1607321|他们两个人都没有犯错误。|1607322|然后我重读你的问题。实际上，答案仍在"Value“属性上。初始化变量(如果好的话)，但如果这样做的话：|1607323|(现在是>=+DateTime.Now)在Linq查询中，您将得到一个错误。应该是(now.Value+>=+DateTime.Now)|1607324|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|Z|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|10|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

That is funny,

I've tried both variation of this code:

<pre><code> System.Nullable&lt;DateTime&gt; now = new System.Nullable&lt;DateTime&gt;();
 now = DateTime.Now;
</code></pre>

and

<pre><code> System.Nullable&lt;DateTime&gt; now;
 now = DateTime.Now;
</code></pre>

and both of them worked without errors.

Then I re-read your question. Actually the answer is still on the "Value" property. Initializing the variable if fine, but if you do:

(now >= DateTime.Now) in the Linq query you will get an error. It should be (now.Value >= DateTime.Now)

blocks|key|1607353|text|这里的问题是，当给定两个不匹配的空值参数时，表达式库将抛出一个异常。这里有一个简单的复制：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1607354|Expression<Func<DateTime?>>+ex1+=+()=>DateTime.Now;
Expression<Func<DateTime>>+ex2+=+()=>DateTime.Now;
var+ex3+=+Expression.GreaterThan(ex1.Body,+ex2.Body);|code-block|syntax|javascript|1607355|我不清楚这是否是一个bug；规则of+C#要求在这种情况下，非空操作数被转换为可空操作数，并且使用了比较的可解除为空的形式。然而，表达式树库不需要来遵循C#的规则，因为表达式树库当然可以用来表示C#表达式、Python表达式、JScript表达式、VB表达式等；它不可能遵循每种语言的所有规则。|offset|length|style|BOLD|1607356|但是无论如何，这看起来可能是个bug，所以我会把它提交给表达式树团队，看看他们怎么说。同时，您可以轻松地通过定义自己的助手方法来解决这个问题，该方法可以修复操作数。一个简短的草图是：|1607357|++++static+Expression+MyGreaterThan(Expression+e1,+Expression+e2)
++++{
++++++++if+(IsNullableType(e1.Type)+&&+!IsNullableType(e2.Type))
++++++++++++e2+=+Expression.Convert(e2,+e1.Type);
++++++++else+if+(!IsNullableType(e1.Type)+&&+IsNullableType(e2.Type))
++++++++++++e1+=+Expression.Convert(e1,+e2.Type);
++++++++return+Expression.GreaterThan(e1,+e2);
++++}
++++static+bool+IsNullableType(Type+t)
++++{
++++++++return+t.IsGenericType+&&+t.GetGenericTypeDefinition()+==+typeof(Nullable<>);
++++}|1607358|但是，注意到，这并不能检查e1和e2的类型是否仅在可空性方面有所不同；如果传入可空int和非空双表达式，则会发生不好的事情。，我将其作为一项练习，以实现更好的逻辑，以检查这两个表达式的类型是否只有空性不同。|1607359|entityMap^0|0|0|G|5|21|5|0|0|0|3|1N|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]|$I|Z|J|10|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|13|8|@$I|14|J|15|K|L]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|T|$]]

The problem here is that the expression library is throwing an exception when given two arguments of mismatched nullability. Here's a simple repro:

<pre><code>Expression&lt;Func&lt;DateTime?&gt;&gt; ex1 = ()=&gt;DateTime.Now;
Expression&lt;Func&lt;DateTime&gt;&gt; ex2 = ()=&gt;DateTime.Now;
var ex3 = Expression.GreaterThan(ex1.Body, ex2.Body);
</code></pre>

It is not clear to me whether this is a bug or not; the rules of C# require that in this scenario, the non-nullable operand is converted to nullable, and the lifted-to-nullable form of the comparison is used. However, the expression tree library is not required to follow the rules of C# because of course the expression tree library can be used to represent C# expressions, Python expressions, JScript expressions, VB expressions and so on; it cannot possibly follow all the rules of every possible language.

But regardless, this looks like it might be a bug, so I'll submit it to the expression tree team and see what they say. In the meanwhile, you can easily work around it by defining your own helper method that fixes the operands up. A quick sketch would be:

<pre><code> static Expression MyGreaterThan(Expression e1, Expression e2)
 {
 if (IsNullableType(e1.Type) &amp;&amp; !IsNullableType(e2.Type))
 e2 = Expression.Convert(e2, e1.Type);
 else if (!IsNullableType(e1.Type) &amp;&amp; IsNullableType(e2.Type))
 e1 = Expression.Convert(e1, e2.Type);
 return Expression.GreaterThan(e1, e2);
 }
 static bool IsNullableType(Type t)
 {
 return t.IsGenericType &amp;&amp; t.GetGenericTypeDefinition() == typeof(Nullable&lt;&gt;);
 }
</code></pre>

However, notice that this does not check that e1 and e2's type differ only in nullability; if you pass in a nullable int and a non-nullable double expression, bad things happen. I leave it as an exercise to implement the better logic that checks whether the two expressions are of type that only differs by nullability.

blocks|key|1607377|text|我找到了一个在.Net框架内工作的解决方案。这里有一个仍在进行中但确实有效的方法，并适用于实体的Linq：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1607378|public+PagedViewModel<T>+Filter<TValue>(Expression<Func<T,+TValue>>+predicate,+FilterType+filterType+=+FilterType.Equals)+{
++++++++var+name+=+(predicate.Body+as+MemberExpression+??+((UnaryExpression)predicate.Body).Operand+as+MemberExpression).Member.Name;++++++++++++
++++++++var+value+=+Expression.Constant(ParamsData[name].To<TValue>(),+typeof+(T).GetProperty(name).PropertyType);++++++++++++++++++++++++

++++++++//+If+nothing+has+been+set+for+filter,+skip+and+don't+filter+data.
++++++++ViewData[name]+=+m_QueryInternal.Distinct(predicate.Compile()).ToSelectList(name,+name,+ParamsData[name]);
++++++++if+(string.IsNullOrWhiteSpace(ParamsData[name]))
++++++++++++return+this;

++++++++var+nameExpression+=+Expression.Parameter(typeof(T),+name);
++++++++var+propertyExpression+=+Expression.Property(nameExpression,+typeof(T).GetProperty(name));

++++++++//+Create+expression+body+based+on+type+of+filter
++++++++Expression+expression;
++++++++MethodInfo+method;
++++++++switch(filterType)+{
++++++++++++case+FilterType.Like:
++++++++++++++++method+=+typeof(string).GetMethod("Contains",+new[]+{+typeof(string)+});
++++++++++++++++expression+=+Expression.Call(propertyExpression,+method,+value);+
++++++++++++++++break;
++++++++++++case+FilterType.EndsWith:
++++++++++++case+FilterType.StartsWith:
++++++++++++++++method+=+typeof(string).GetMethod(filterType.ToString(),+new[]+{+typeof(string)+});
++++++++++++++++expression+=+Expression.Call(propertyExpression,+method,+value);
++++++++++++++++break;
++++++++++++case+FilterType.GreaterThan:
++++++++++++++++expression+=+Expression.GreaterThan(propertyExpression,+value);++++++++++++++++++++
++++++++++++++++break;
++++++++++++case+FilterType.Equals:
++++++++++++++++expression+=+Expression.Equal(propertyExpression,+value);
++++++++++++++++break;
++++++++++++default:
++++++++++++++++throw+new+ArgumentException("Filter+Type+could+not+be+determined");
++++++++}++++++++++++

++++++++//+Execute+the+expression+against+Query.
++++++++var+methodCallExpression+=+Expression.Call(
++++++++++++typeof+(Queryable),
++++++++++++"Where",
++++++++++++new[]+{+Query.ElementType+},
++++++++++++Query.Expression,
++++++++++++Expression.Lambda<Func<T,+bool>>(expression,+new[]+{+nameExpression+}));

++++++++//+Filter+the+current+Query+data.
++++++++Query+=+Query.Provider.CreateQuery<T>(methodCallExpression);++++++++++++

++++++++return+this;
++++}|code-block|syntax|javascript|1607379|这里发生的是将表达式常量值转换为谓词类型。为了清楚起见，以下方法调用了该方法：|1607380|var+paramsData+=+new+NameValueCollection+{+{+"CreatedOn",+DateTime.Today.ToString()+}+};
++++++++var+model+=+m_data.ToPagedList(new+ViewDataDictionary(),+paramsData,+1,+10,+null,+x+=>+x.LastName)
++++++++++++++++++++++++++.Filters(Criteria<TrainerProfile>.New(x+=>+x.CreatedOn,+FilterType.GreaterThan))
++++++++++++++++++++++++++.Setup();|1607381|这段代码是基于MVC+3的，因此出现了一些ParamsData“is”(Request.Params)。|1607382|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I found a solution that works within the .Net framework. Here a method that's still in progress but it does work, and works for Linq to Entities:

<pre><code>public PagedViewModel&lt;T&gt; Filter&lt;TValue&gt;(Expression&lt;Func&lt;T, TValue&gt;&gt; predicate, FilterType filterType = FilterType.Equals) {
 var name = (predicate.Body as MemberExpression ?? ((UnaryExpression)predicate.Body).Operand as MemberExpression).Member.Name; 
 var value = Expression.Constant(ParamsData[name].To&lt;TValue&gt;(), typeof (T).GetProperty(name).PropertyType); 

 // If nothing has been set for filter, skip and don't filter data.
 ViewData[name] = m_QueryInternal.Distinct(predicate.Compile()).ToSelectList(name, name, ParamsData[name]);
 if (string.IsNullOrWhiteSpace(ParamsData[name]))
 return this;

 var nameExpression = Expression.Parameter(typeof(T), name);
 var propertyExpression = Expression.Property(nameExpression, typeof(T).GetProperty(name));

 // Create expression body based on type of filter
 Expression expression;
 MethodInfo method;
 switch(filterType) {
 case FilterType.Like:
 method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
 expression = Expression.Call(propertyExpression, method, value); 
 break;
 case FilterType.EndsWith:
 case FilterType.StartsWith:
 method = typeof(string).GetMethod(filterType.ToString(), new[] { typeof(string) });
 expression = Expression.Call(propertyExpression, method, value);
 break;
 case FilterType.GreaterThan:
 expression = Expression.GreaterThan(propertyExpression, value); 
 break;
 case FilterType.Equals:
 expression = Expression.Equal(propertyExpression, value);
 break;
 default:
 throw new ArgumentException("Filter Type could not be determined");
 } 

 // Execute the expression against Query.
 var methodCallExpression = Expression.Call(
 typeof (Queryable),
 "Where",
 new[] { Query.ElementType },
 Query.Expression,
 Expression.Lambda&lt;Func&lt;T, bool&gt;&gt;(expression, new[] { nameExpression }));

 // Filter the current Query data.
 Query = Query.Provider.CreateQuery&lt;T&gt;(methodCallExpression); 

 return this;
 }
</code></pre>

What happens here is the Expression Constant value is cast to the predicate type. For clarity this method is called by:

<pre><code>var paramsData = new NameValueCollection { { "CreatedOn", DateTime.Today.ToString() } };
 var model = m_data.ToPagedList(new ViewDataDictionary(), paramsData, 1, 10, null, x =&gt; x.LastName)
 .Filters(Criteria&lt;TrainerProfile&gt;.New(x =&gt; x.CreatedOn, FilterType.GreaterThan))
 .Setup();
</code></pre>

This code is MVC 3 based, hence some of the ParamsData[""] which is (Request.Params).

blocks|key|1576510|text|这是相当容易的，不需要长时间的解释，因为我看到了。除此之外的一切都是没有必要的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1576511|Expression.Constant()接受一个Type参数。|offset|length|style|CODE|1576512|例如，使用int来执行int？而不是Expression.Constant(someInt)，使用Expression.Constant(someInt,+typeof(int?))，这将把someInt转换为int？在执行之前。|1576513|如果需要，可以为您的类型使用泛型和/或反射。|1576514|这将是使用泛型和反射创建GreaterThan表达式的完整代码。使用这个，它是否为空并不重要。|1576515|var+greaterThan+=+Expression.GreaterThan(
++++++++Expression.Property(Expression.Parameter(typeof(T)),+property),
++++++++Expression.Constant(inValue,+property.PropertyType));|code-block|syntax|javascript|1576516|entityMap^0|0|0|L|P|4|0|I|S|1D|16|2N|7|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@$D|11|E|12|F|G]|$D|13|E|14|F|G]|$D|15|E|16|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|17|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|18|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|19|8|@]|9|@]|A|$Q|R]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$]]

This is pretty easy and doesn't need a long explaination as from what I have seen. Everything beyond this is uneccessary.
<code>Expression.Constant()</code> takes a <code>Type</code> parameter.
So for example, with ints to int?. Instead of <code>Expression.Constant(someInt)</code> use <code>Expression.Constant(someInt, typeof(int?))</code> This will cast <code>someInt</code> to int? before executing.
Feel free to use generics and/or reflection for your types if need be.
This would be full code to create a GreaterThan expression using generics and reflection. Using this, it doesn't matter if it is nullable or not
<pre><code>var greaterThan = Expression.GreaterThan(
 Expression.Property(Expression.Parameter(typeof(T)), property),
 Expression.Constant(inValue, property.PropertyType));
</code></pre>

I am creating some dynamic linq and am having problems with the following exception:

<blockquote>
 The binary operator GreaterThanOrEqual
 is not defined for the types
 'System.Nullable`1[System.DateTime]'
 and 'System.DateTime'
</blockquote>

I get why, because my field type is nullable and Im passing in DateTime.Now essentially.

So in trying to resolve this issue I've tried

<pre><code>System.Nullable&lt;DateTime&gt; now;
now = DateTime.Now;
</code></pre>

But the resulting type is a non-nullable object and hence still giving me the above exception.

Any suggestions?!

Update: For more clarification the now variable becomes a non-nullable type when it is set rather than staying as a nullable DateTime so the match throws an exception

Update: The actual code can be seen in the CodePlex project:

<a href="http://webquarters.codeplex.com/SourceControl/changeset/view/36529#574700" rel="noreferrer">http://webquarters.codeplex.com/SourceControl/changeset/view/36529#574700</a>

The offending line is ~145 

<pre><code>fExp = Expression.GreaterThanOrEqual(fExpLeft, fExpRight);
</code></pre>

Expression.GreaterThan fails if one operand is nullable type, other is non-nullable

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我正在创建一些动态linq，并遇到以下问题：二进制运算符GreaterThanOrEqual未为“System.Nullable`1System.DateTime”和“System.DateTime”类型定义我明白为什么，因为我的字段类型是可空的，并且本质上是传入DateTime.Now的。所以在试图解决这个问题时，我...

问如果一个操作数为可空类型，另一个操作数为不可空类型，则Expression.GreaterThan将失败。
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问如果一个操作数为可空类型，另一个操作数为不可空类型，则Expression.GreaterThan将失败。EN