[英]How do I combine two Expressions?
我正在尝试构建一个将应用于IQueryable集合的表达式。
我可以构建一个这样的表达式:
[TestClass]
public class ExpressionTests
{
private IQueryable<MyEntity> entities;
private class MyEntity
{
public string MyProperty { get; set; }
}
[TestInitialize]
public void Setup()
{
entities = new[]
{
new MyEntity {MyProperty = "first"},
new MyEntity {MyProperty = "second"}
}.AsQueryable();
}
[TestMethod]
public void TestQueryingUsingSingleExpression()
{
Expression<Func<MyEntity, bool>> expression = e => e.MyProperty.Contains("irs");
Assert.AreEqual(1, entities.Where(expression).Count());
}
}
现在我想分开表达式的两个部分:
[TestMethod]
public void TestQueryingByCombiningTwoExpressions()
{
Expression<Func<MyEntity, string>> fieldExpression = e => e.MyProperty;
Expression<Func<string, bool>> operatorExpression = e => e.Contains("irs");
// combine the two expressions somehow...
Expression<Func<MyEntity, bool>> combinedExpression = ???;
Assert.AreEqual(1, entities.Where(combinedExpression).Count());
}
有关如何做到这一点的任何建议?
Btw将解析表达式的提供程序是Linq for NHibernate。
看看你的两个表达式树:
| | Lambda Lambda / \ / \ / \ / \ Property Parameter x Call Parameter y / \ / | \ / \ / | \ x MyProperty EndsWidth y Constant | "5"
您需要创建一个如下所示的新树:
| Lambda / \ / \ Call Parameter z / | \ / | \ EndsWith | Constant | \ Property "5" / \ / \ z MyProperty
您可以轻松地看到新树的哪些部分来自哪个原始树。
要创建树,可以使用第二个lambda表达式(Call)的主体,并将所有出现的y
替换为第一个lambda表达式(Property)的主体,并将所有出现的x
替换为z
。 然后将结果包装在带有参数z
的新lambda表达式中。
您可以使用ExpressionVisitor类重写树,使用Expression.Lambda方法创建新的lambda表达式。
这取决于提供商支持的内容; 如果它支持子表达式(LINQ-to-SQL没有,EF不支持;我不知道NH),那么:
var combinedExpression = Expression.Lambda<Func<MyEntity, bool>>(
Expression.Invoke(operatorExpression, fieldExpression.Body),
fieldExpression.Parameters);
但是,如果不是,则需要使用ExpressionVisitor
来合并它们。
根据dtb和Marc的建议,我使用ExpressionVisitor重写表达式树,这是我能够管理的最干净的:
public class ExpressionBuilder<T> : ExpressionVisitor where T : class
{
private Expression fieldExpressionBody;
protected override Expression VisitParameter(ParameterExpression node)
{
return fieldExpressionBody;
}
public Expression<Func<T, bool>> Build(
Expression<Func<T, string>> fieldExpression,
Expression<Func<string, bool>> operatorExpression)
{
fieldExpressionBody = fieldExpression.Body;
Expression newExpressionBody = Visit(operatorExpression.Body);
return Expression.Lambda<Func<T, bool>>(newExpressionBody, fieldExpression.Parameters[0]);
}
}
并在我的单元测试中使用它:
[TestMethod]
public void TestQueryingByCombiningTwoExpressions()
{
Expression<Func<MyEntity, string>> fieldExpression = e => e.MyProperty;
Expression<Func<string, bool>> operatorExpression = o => o.Contains("irs");
var builder = new ExpressionBuilder<MyEntity>();
Expression<Func<MyEntity, bool>> combinedExpression = builder.Build(fieldExpression, operatorExpression);
Assert.AreEqual(1, entities.Where(combinedExpression).Count());
Assert.AreEqual("e => e.MyProperty.Contains(\"irs\")", combinedExpression.ToString());
}
完成之后,这是一个可以使用2,3或n个表达式的版本
public class ExpressionMerger : ExpressionVisitor
{
Expression CurrentParameterExpression { get; set; }
public Expression<Func<TIn, TOut>> Merge<TIn, TA, TOut>(Expression<Func<TIn, TA>> inner, Expression<Func<TA, TOut>> outer)
{
return MergeAll<TIn, TOut>(inner, outer);
}
public Expression<Func<TIn, TOut>> Merge<TIn, TA, TB, TOut>(Expression<Func<TIn, TA>> inner, Expression<Func<TA, TB>> transition, Expression<Func<TB, TOut>> outer)
{
return MergeAll<TIn, TOut>(inner, transition, outer);
}
protected Expression<Func<TIn, TOut>> MergeAll<TIn, TOut>(params LambdaExpression[] expressions)
{
CurrentParameterExpression = expressions[0].Body;
foreach (var expression in expressions.Skip(1))
{
CurrentParameterExpression = Visit(expression.Body);
}
return Expression.Lambda<Func<TIn, TOut>>(CurrentParameterExpression, expressions[0].Parameters[0]);
}
protected override Expression VisitParameter(ParameterExpression node)
{
//replace current lambda parameter with ~previous lambdas
return CurrentParameterExpression;
}
}
}
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