blocks|key|193695|text|您不能这样做，因为EF不知道如何将Location上的相等比较转换为SQL表达式。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|193696|但是，如果您知道要比较Location的哪些属性，则可以使用匿名类型进行比较：|193697|var+location+=+GetCurrentLocation();
var+locationObj+=+new+{+location.LocationName,+location.LocationDescription+};
employees+=+DataContext.Employees.Where(e+=>+new+{+e.Location.LocationName,+e.Location.Description+}+==+locationObj);|code-block|syntax|javascript|193698|当然，这相当于：|193699|var+location+=+GetCurrentLocation();
employees+=+DataContext.Employees.Where(e+=>+e.Location.LocationName+==+location.Name+&&+
+++++++++++++++++++++++++++++++++++++++++++++e.Location.Description+==+location.Description);|193700|entityMap^0|H|8|0|B|8|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|R|$]]

You can't do that because EF doesn't know how to translate equality comparisons on <code>Location</code> into a SQL expression. 

However, if you know what properties of <code>Location</code> you want to compare, you can do this with anonymous types:

<pre><code>var location = GetCurrentLocation();
var locationObj = new { location.LocationName, location.LocationDescription };
employees = DataContext.Employees.Where(e =&gt; new { e.Location.LocationName, e.Location.Description } == locationObj);
</code></pre>

Of course that's equivalent to:

<pre><code>var location = GetCurrentLocation();
employees = DataContext.Employees.Where(e =&gt; e.Location.LocationName == location.Name &amp;&amp; 
 e.Location.Description == location.Description);
</code></pre>

blocks|key|191248|text|运行下面的代码。我想测试一下您的假设，即e+=>+e.Location+==+location正在编译成可以用Expression.Equal、Expression.Property和Expression.Constant构建的东西。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|191249|++++class+Program+{
+++++++static+void+Main(string[]+args)+{
++++++++++var+location+=+new+Location();
++++++++++Expression<Func<Employee,+bool>>+expression+=+e+=>+e.Location+==+location;

++++++++++var+untypedBody+=+expression.Body;

++++++++++//The+untyped+body+is+a+BinaryExpression
+++++++++++Debug.Assert(
++++++++++++++typeof(BinaryExpression).IsAssignableFrom(untypedBody.GetType()),+
++++++++++++++"Not+Expression.Equal");

+++++++++++var+body+=+(BinaryExpression)untypedBody;
+++++++++++var+untypedLeft+=+body.Left;
+++++++++++var+untypedRight+=+body.Right;

+++++++++++//The+untyped+left+expression+is+a+MemberExpression
+++++++++++Debug.Assert(
++++++++++++++typeof(MemberExpression).IsAssignableFrom(untypedLeft.GetType()),+
++++++++++++++"Not+Expression.Property");

+++++++++++////The+untyped+right+expression+is+a+ConstantExpression
++++++++++//Debug.Assert(
++++++++++//+++typeof(ConstantExpression).IsAssignableFrom(untypedRight.GetType()),+++++++++++++++++
++++++++++//+++"Not+Expression.Constant");

++++++++++//The+untyped+right+expression+is+a+MemberExpression?
++++++++++Debug.Assert(
+++++++++++++++typeof(MemberExpression).IsAssignableFrom(untypedRight.GetType())));
++++}
}

public+class+Employee
{
++++public+Location+Location+{+get;+set;+}
}

public+class+Location+{+}|code-block|syntax|javascript|191250|看起来不是，这是因为正确的表达式不是常量。要查看这一点，请取消注释掉的代码。|191251|我不明白的是为什么正确的表达式是MemberExpression。也许了解linq表达式编译器的人可以比我更深入地了解这一点。|191252|编辑:这可能与lambdas中的闭包有关-一个类是在幕后创建的，其中包含闭合的变量。然后，该位置可能是该类的成员。我不确定这一点，但这是我怀疑的。|191253|This+post可能会为这种情况提供更多信息。|offset|length|191254|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/3991621/can-i-capture-a-local-variable-into-a-linq-expression-as-a-constant-rather-than^0|0|0|0|0|0|0|9|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|13|8|@]|9|@$O|14|P|15|1|16]]|A|$]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Give the code below a run. I wanted to test your assumption that e => e.Location == location is compiling into something that can be constructed with Expression.Equal, Expression.Property, and Expression.Constant.

<pre><code> class Program {
 static void Main(string[] args) {
 var location = new Location();
 Expression&lt;Func&lt;Employee, bool&gt;&gt; expression = e =&gt; e.Location == location;

 var untypedBody = expression.Body;

 //The untyped body is a BinaryExpression
 Debug.Assert(
 typeof(BinaryExpression).IsAssignableFrom(untypedBody.GetType()), 
 "Not Expression.Equal");

 var body = (BinaryExpression)untypedBody;
 var untypedLeft = body.Left;
 var untypedRight = body.Right;

 //The untyped left expression is a MemberExpression
 Debug.Assert(
 typeof(MemberExpression).IsAssignableFrom(untypedLeft.GetType()), 
 "Not Expression.Property");

 ////The untyped right expression is a ConstantExpression
 //Debug.Assert(
 // typeof(ConstantExpression).IsAssignableFrom(untypedRight.GetType()), 
 // "Not Expression.Constant");

 //The untyped right expression is a MemberExpression?
 Debug.Assert(
 typeof(MemberExpression).IsAssignableFrom(untypedRight.GetType())));
 }
}

public class Employee
{
 public Location Location { get; set; }
}

public class Location { }
</code></pre>

It seems like it isn't, and its because the right expression isn't a Constant. To see this, uncomment the commented out code.

What I don't understand is why the right expression is a MemberExpression. Perhaps someone who knows the linq expression compiler can shed more light onto this then I can.

Edit: This may have to do with closure in lambdas - a class is created behind the scenes which contains the closed over variables. The location might then be a member of that class. I'm not sure about this, but it's what I suspect.

<a href="https://stackoverflow.com/questions/3991621/can-i-capture-a-local-variable-into-a-linq-expression-as-a-constant-rather-than">This post</a> may shed additional light on the situation.

blocks|key|191576|text|除了在前面的答案中提到的之外。一个更具体的解决方案是这样的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|191577|public+static+Expression+CreateExpression<T>(string+propertyName,+object+valueToCompare)
{
++++//+get+the+type+of+entity
++++var+entityType+=+typeof(T);
++++//+get+the+type+of+the+value+object
++++var+valueType+=+valueToCompare.GetType();
++++var+entityProperty+=+entityType.GetProperty(propertyName);
++++var+propertyType+=+entityProperty.PropertyType;


++++//+Expression:+"entity"
++++var+parameter+=+Expression.Parameter(entityType,+"entity");

++++//+check+if+the+property+type+is+a+value+type
++++//+only+value+types+work+
++++if+(propertyType.IsValueType+%7C%7C+propertyType.Equals(typeof(string)))
++++{
++++++++//+Expression:+entity.Property+==+value
++++++++return+Expression.Equal(
++++++++++++Expression.Property(parameter,+entityProperty),
++++++++++++Expression.Constant(valueToCompare)
++++++++);
++++}
++++//+if+not,+then+use+the+key
++++else
++++{
++++++++//+get+the+key+property
++++++++var+keyProperty+=+propertyType.GetProperties().FirstOrDefault(p+=>+p.GetCustomAttributes(typeof(KeyAttribute),+false).Length+>+0);

++++++++//+Expression:+entity.Property.Key+==+value.Key
++++++++return+Expression.Equal(
++++++++++++Expression.Property(
++++++++++++++++Expression.Property(parameter,+entityProperty),
++++++++++++++++keyProperty
++++++++++++),
++++++++++++Expression.Constant(
++++++++++++++++keyProperty.GetValue(valueToCompare),
++++++++++++++++keyProperty.PropertyType
++++++++++++)
++++++++);
++++}
}|code-block|syntax|javascript|191578|要点:|offset|length|style|BOLD|191579|191580|确保检查KeyAttribute)|ordered-list-item|CODE|191581|This代码是否为空，并确保|191582|+propertyType和valueType兼容(它们可能是同一类型，也可能是此处所做的nulls假设)(例如，您确实分配了一个KeyAttribute)|191583|This代码未经过测试，因此它并未完全准备好复制/粘贴。|191584|191585|希望这能有所帮助。|191586|entityMap^0|0|0|0|3|0|0|4|C|0|0|1|C|E|9|1T|C|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@$I|15|J|16|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|17|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|18|8|@$I|19|J|1A|K|Q]]|9|@]|A|$]]|$1|R|3|S|5|P|7|1B|8|@]|9|@]|A|$]]|$1|T|3|U|5|P|7|1C|8|@$I|1D|J|1E|K|Q]|$I|1F|J|1G|K|Q]|$I|1H|J|1I|K|Q]]|9|@]|A|$]]|$1|V|3|W|5|P|7|1J|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1L|8|@]|9|@]|A|$]]|$1|10|3|-4|5|6|7|1M|8|@]|9|@]|A|$]]]|11|$]]

In addition to what has been mentioned in previous answers. A more specific solution would go as such:

<pre><code>public static Expression CreateExpression&lt;T&gt;(string propertyName, object valueToCompare)
{
 // get the type of entity
 var entityType = typeof(T);
 // get the type of the value object
 var valueType = valueToCompare.GetType();
 var entityProperty = entityType.GetProperty(propertyName);
 var propertyType = entityProperty.PropertyType;


 // Expression: "entity"
 var parameter = Expression.Parameter(entityType, "entity");

 // check if the property type is a value type
 // only value types work 
 if (propertyType.IsValueType || propertyType.Equals(typeof(string)))
 {
 // Expression: entity.Property == value
 return Expression.Equal(
 Expression.Property(parameter, entityProperty),
 Expression.Constant(valueToCompare)
 );
 }
 // if not, then use the key
 else
 {
 // get the key property
 var keyProperty = propertyType.GetProperties().FirstOrDefault(p =&gt; p.GetCustomAttributes(typeof(KeyAttribute), false).Length &gt; 0);

 // Expression: entity.Property.Key == value.Key
 return Expression.Equal(
 Expression.Property(
 Expression.Property(parameter, entityProperty),
 keyProperty
 ),
 Expression.Constant(
 keyProperty.GetValue(valueToCompare),
 keyProperty.PropertyType
 )
 );
 }
}
</code></pre>

IMPORTANT POINTS :

<ol>
<li>Make sure to check for nulls</li>
<li>Make sure <code>propertyType</code> and <code>valueType</code> are compatible (either they are the same type or are convertible)</li>
<li>Several assumptions are made here (e.g. that you do assign a <code>KeyAttribute</code>)</li>
<li>This code is not tested, so it is not exactly copy/paste ready.</li>
</ol>

Hope that helps.

Given a primitive value <code>age</code> I know how to create an expression like this:

<pre><code>//assuming: age is an int or some other primitive type
employee =&gt; employee.Age == age
</code></pre>

By doing this:

<pre><code>var entityType = typeof(Employee);
var propertyName = "Age";
int age = 30;
var parameter = Expression.Parameter(entityType, "entity");

var lambda = Expression.Lambda(
 Expression.Equal(
 Expression.Property(parameter, propertyName),
 Expression.Constant(age)
 ) 
 , parameter);
</code></pre>

That works fine except in scenarios where the property and constant in question are not primitive types.

How would I construct a similar expression if the comparison is between objects?

With EF I can just write:

<pre><code>Location location = GetCurrentLocation();
employees = DataContext.Employees.Where(e =&gt; e.Location == location);
</code></pre>

That also works, but if I try to create the same expression:

<pre><code>var entityType = typeof(Employee);
var propertyName = "Location";
var location = GetCurrentLocation();
var parameter = Expression.Parameter(entityType, "entity");

var lambda = Expression.Lambda(
 Expression.Equal(
 Expression.Property(parameter, propertyName),
 Expression.Constant(location)
 ) 
 , parameter);
</code></pre>

I get an error that says:

<code>Unable to create a constant value of type 'Location'. Only primitive types or enumeration types are supported in this context.</code>

My suspicion is that <code>Expression.Constant()</code> only expects primitive types, so I need to use a different expression factory method. (maype <code>Expression.Object</code>? - I know that doesn't exist)

Is there a way to create an expression that compares objects? Why is that EF is able to interpret it correctly if its a compiled LINQ statement, but not when it is an expression?

Creating a LINQ Expression where parameter equals object

给定一个原始值age，我知道如何创建这样的表达式：//assuming: age is an int or some other primitive typeemployee => employee.Age == age通过这样做：var entityType = typeof(Employee);var proper...

问创建参数等于对象的LINQ表达式
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