[英]IQueryable expression type mismatch
I want to create an extension method for a LINQ expression but I'm stuck. 我想为LINQ表达式创建扩展方法,但是我遇到了麻烦。 What I need is just to create a method which will add a specific Where clause to a
Queryable
. 我需要的只是创建一个将特定的Where子句添加到
Queryable
。 Something like: 就像是:
var hierarchy = "a string";
Session.Query<SomeClass>.Where(x => x.Layer.Hierarchy.StartsWith(hierarchy) ||
x.Layer.Hierarchy == hierarchy);
to become: 成为:
var hierarchy = "a string";
Session.Query<SomeClass>.LayerHierarchy(x => x.Layer, hierarchy);
And do that Where logic inside. 并执行其中的逻辑。 So basicly the extension method
LayerHierarchy()
is running over the Queryable
of T
but the subject is of type Layer
: 因此,基本上,扩展方法
LayerHierarchy()
在T
的Queryable
之上运行,但是主题的类型为Layer
:
public static IQueryable<T> LayerHierarchy<T>(this IQueryable<T> query,
Expression<Func<T, Layer>> layer,
string hierarchy)
{
var parameterExp = Expression.Parameter(typeof(Layer), "layer");
var propertyExp = Expression.Property(parameterExp, "Hierarchy");
// StartWith method
MethodInfo methodStartsWith = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
var valueStartsWith = Expression.Constant(string.Concat(hierarchy, "|"), typeof(string));
var methodExpStartsWith = Expression.Call(propertyExp, methodStartsWith, valueStartsWith);
var startsWith = Expression.Lambda<Func<Layer, bool>>(methodExpStartsWith, parameterExp);
// Equals method
MethodInfo methodEquals = typeof(string).GetMethod("Equals", new[] { typeof(string) });
var valueEquals = Expression.Constant(hierarchy, typeof(string));
var methodExpEquals = Expression.Call(propertyExp, methodEquals, valueEquals);
var equals = Expression.Lambda<Func<Layer, bool>>(methodExpEquals, parameterExp);
return query
.Where(startsWith)
.Where(equals);
}
Everything works fine above the return
line. 在
return
线以上,一切正常。 It complains that... 它抱怨...
Cannot convert from
System.Linq.Expressions.Expression<System.Func<Layer, bool>>
toSystem.Linq.Expressions.Expression<System.Func<T, int, bool>>
无法从
System.Linq.Expressions.Expression<System.Func<Layer, bool>>
为System.Linq.Expressions.Expression<System.Func<T, int, bool>>
when trying to pass the expressions to query.Where()
method. 尝试将表达式传递给
query.Where()
方法时。 How can I fix it? 我该如何解决?
Well, the problem is how you are creating the Lambda
s. 好吧,问题在于您如何创建
Lambda
。 They should begin from T
, not from Layer
: 它们应该从
T
开始,而不是从Layer
:
var startsWith = Expression.Lambda<Func<T, bool>>(methodExpStartsWith, parameterExp);
var equals = Expression.Lambda<Func<T, bool>>(methodExpEquals, parameterExp);
However, in order for this to work, you are missing one more PropertyExpression
. 但是,为了使其正常工作,您还缺少一个
PropertyExpression
。
Your query now looks like: 您的查询现在看起来像:
(Layer)x => x.Hierarchy.StartsWith(...)
(层)x => x.Hierarchy.StartsWith(...)
When, what you want is this: 什么时候,您想要的是:
(T)x => x.Layer.Hierarchy.StartsWith(...)
(T)x => x.Layer.Hierarchy.StartsWith(...)
So, use this instead: 因此,请改用此:
var parameterExp = Expression.Parameter(typeof(T), "item");
var layerExp = Expression.Property(parameterExp, "Layer");
var propertyExp = Expression.Property(layerExp, "Hierarchy");
Your logic should change a little though, since two .Where
will generate an AND
condition between them, and it seems like you want one of them to be true ( StartsWith
or Equals
), so: 但是,您的逻辑应该稍有变化,因为两个
.Where
会在它们之间生成一个AND
条件,并且似乎您希望其中一个为真( StartsWith
或Equals
),因此:
var parameterExp = Expression.Parameter(typeof(T), "item");
var layerExp = Expression.Property(parameterExp, "Layer");
var propertyExp = Expression.Property(layerExp, "Hierarchy");
// StartWith method
MethodInfo methodStartsWith = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
var valueStartsWith = Expression.Constant(string.Concat(hierarchy, "|"), typeof(string));
var methodExpStartsWith = Expression.Call(propertyExp, methodStartsWith, valueStartsWith);
// Equals method
MethodInfo methodEquals = typeof(string).GetMethod("Equals", new[] { typeof(string) });
var valueEquals = Expression.Constant(hierarchy, typeof(string));
var methodExpEquals = Expression.Call(propertyExp, methodEquals, valueEquals);
var orElseExp = Expression.OrElse(methodExpStartsWith, methodExpEquals);
var orElse = Expression.Lambda<Func<T, bool>>(orElseExp, parameterExp);
return query.Where(orElse);
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