问题描述
给出一个简单的父/子类结构。我想用linqkit到父应用子拉姆达前pression。我也想通过一个实用的方法来提供拉姆达前pression。
公共类Foo
{
公共酒吧酒吧{搞定;组; }
}公共类酒吧
{
公共字符串值{获得;组; }
公共静态防爆pression<&Func键LT;酒吧,布尔>> GetLambdaX()
{
返回C => c.Value ==A;
}
}
...防爆pression<&Func键LT;富,布尔>> LX = C => 。Bar.GetLambdaX()调用(c.Bar);
Console.WriteLine(lx.Expand());
以上code抛出
System.InvalidCastException:无法投类型的对象
System.Linq.Ex pressions.MethodCallEx pression到类型
System.Linq.Ex pressions.LambdaEx pression。
在LinqKit.Ex pressionExpander.VisitMethodCall(MethodCallEx pression米)
在LinqKit.Ex pressionVisitor.Visit(前pression EXP)
在LinqKit.Ex pressionVisitor.VisitLambda(LambdaEx pression拉姆达)
在LinqKit.Ex pressionVisitor.Visit(前pression EXP)
在LinqKit.Extensions.Expand< TDelegate>(防爆pression`1表达式)
VAR lambdaX = Bar.GetLambdaX();防爆pression<&Func键LT;富,布尔>> LX = C => lambdaX.Invoke(c.Bar);
这工作。
小心写
防爆pression<&Func键LT;富,布尔>> LX = ...
分配编译器处理该行之前和你的情况创建了一个方法调用前pression(即调用Bar.GetLambdaX())分配给LX。
如果您使用VAR ...然后Bar.GetLambdaX();叫和分配,其值(这是拉姆达前pression)在LX = ...
稍后用于Given a simple parent/child class structure. I want to use linqkit to apply a child lambda expression on the parent. I also want the Lambda expression to be provided by a utility method.
public class Foo
{
public Bar Bar { get; set; }
}
public class Bar
{
public string Value { get; set; }
public static Expression<Func<Bar, bool>> GetLambdaX()
{
return c => c.Value == "A";
}
}
...
Expression<Func<Foo, bool>> lx = c => Bar.GetLambdaX().Invoke(c.Bar);
Console.WriteLine(lx.Expand());
The above code throws
System.InvalidCastException: Unable to cast object of type
'System.Linq.Expressions.MethodCallExpression' to type
'System.Linq.Expressions.LambdaExpression'.
at LinqKit.ExpressionExpander.VisitMethodCall(MethodCallExpression m)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.ExpressionVisitor.VisitLambda(LambdaExpression lambda)
at LinqKit.ExpressionVisitor.Visit(Expression exp)
at LinqKit.Extensions.Expand<TDelegate>(Expression`1 expr)
var lambdaX = Bar.GetLambdaX();
Expression<Func<Foo, bool>> lx = c => lambdaX.Invoke(c.Bar);
This works.
Be careful with writing
Expression<Func<Foo, bool>> lx = ...
Before assignment the compiler processes this line and in your case creates a Method Call Expression (i.e. for calling Bar.GetLambdaX()) that is assigned to lx.
If you use var ... then Bar.GetLambdaX(); is called and assigned, the value of which (which is the lambda expression) is used later in lx = ...
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