[英]How to combine result of multiple Linq Expressions into one Expression?
I have a list of Linq Expressions List<Expression>
where each expression type (the type the expression would return) is either Item
or Item[]
. 我有一个Linq Expressions
List<Expression>
列表,其中每个表达式类型(表达式将返回的类型)是Item
或Item[]
。
I'm trying to write some code that would take mentioned collection as an input parameter and produce a Linq Expression that would return one list (or array) of items ( Item[]
). 我正在尝试编写一些代码,这些代码将提到的collection作为输入参数,并生成一个Linq表达式,该表达式将返回项的一个列表(或数组)(
Item[]
)。
Here is an abstract example: 这是一个抽象的例子:
public static string[] GetStrings()
{
return new[]
{
"first",
"second",
"third"
};
}
public static string GetString()
{
return "single1";
}
private void SOExample()
{
var expressions = new List<Expression>
{
Expression.Call(GetType().GetMethod("GetString")),
Expression.Call(GetType().GetMethod("GetStrings")),
Expression.Call(GetType().GetMethod("GetString")),
Expression.Call(GetType().GetMethod("GetStrings"))
};
// some magic code here
var combined = SomeMagicHere(expressions);
}
private Expression SomeMagicHere(List<Expression> expressions)
{
foreach (var expression in expressions)
{
if (expression.Type.IsArray)
{
// Use array's elements
}
else
{
// Use expression
}
}
What I'm trying to make is to produce one Expression that would return a list of Item
(strings in my example) from provided list. 我想做的是生成一个表达式,该表达式将从提供的列表中返回一个
Item
列表(在我的示例中为字符串)。
This seems a very odd scenario, and in most cases I would expect to see use of raw reflection (or maybe delegates) rather than Expression
here - it isn't an obvious fit. 这似乎是一个非常奇怪的情况,在大多数情况下,我希望看到此处使用原始反射(或委托)而不是
Expression
这显然不是合适的。 But: you can do that by turning the expressions into a block that each call Add
or AddRange
to append values into the list. 但是:您可以通过将表达式变成一个块来实现 ,每个调用
Add
或AddRange
将值追加到列表中。 For example: 例如:
using System;
using System.Collections.Generic;
using System.Linq.Expressions;
static class Program
{
public static string GetString()
{
return "single1";
}
public static string[] GetStrings()
{
return new[]
{
"first",
"second",
"third"
};
}
static void Main()
{
var expressions = new List<Expression>
{
Expression.Call(typeof(Program).GetMethod("GetString")),
Expression.Call(typeof(Program).GetMethod("GetStrings")),
Expression.Call(typeof(Program).GetMethod("GetString")),
Expression.Call(typeof(Program).GetMethod("GetStrings"))
};
// some magic code here
var combined = SomeMagicHere(expressions);
// show it works
var lambda = Expression.Lambda<Func<List<string>>>(combined);
var list = lambda.Compile()();
}
private static Expression SomeMagicHere(List<Expression> expressions)
{
List<Expression> blockContents = new List<Expression>();
var var = Expression.Variable(typeof(List<string>), "list");
blockContents.Add(Expression.Assign(var,
Expression.New(typeof(List<string>))));
foreach (var expression in expressions)
{
if (expression.Type.IsArray)
{
blockContents.Add(Expression.Call(
var, var.Type.GetMethod("AddRange"), expression));
}
else
{
blockContents.Add(Expression.Call(var,
var.Type.GetMethod("Add"), expression));
}
}
blockContents.Add(var); // last statement in a block is the effective
// value of the block
return Expression.Block(new[] {var}, blockContents);
}
}
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