使用表达式树的 Foreach 循环

    public class TestObject
    {
        public TestObject()
        {
            ClassList = new List<Class>();
        }
        public int Age { get; set; }
        public List<Class> ClassList { get; set; } 
    }
    public class Class
    {
        public string Name { get; set; }
        public int ClassId { get; set; }
    }

//type==TestObject at runtime
//propertyName == "ClassList"
   ParameterExpression recordExpression = Expression.Parameter(type, "record");
   memberExpression = MemberExpression.Property(recordExpression, propertyName);
   Type getEnumerableDelegateType =
                typeof(Func<,>).MakeGenericType(new Type[] { type, memberExpression.Type}); 
   var getList = Expression.Lambda(getEnumerableDelegateType, memberExpression, recordExpression);

   var getListFunc = new Func<TestObject, List<Class>>((TestObject obj1) => obj1.ClassList);
   Action<List<Class>> listAction = delegate(List<Class> data)
                {
                    foreach (var dataChannelWithUnitse in data)
                    {
                        //Apply generated delegate
                    }
                };
     Action<TestObject> overallAction = delegate(TestObject data)
                {
                    var x = getListFunc.Invoke(data);
                    listAction.Invoke(x as List<Class>);
                };

    var typeParam = Expression.Parameter(type, "Input");
    var listVariable = Expression.Variable(memberExpression.Type, "List");
    var enumerator = Expression.Variable(typeof(IEnumerator<>).MakeGenericType(dataType));

    var enumeratorType = typeof(IEnumerator<>).MakeGenericType(dataType);
    var enumerableType = typeof(IEnumerable<>).MakeGenericType(dataType);
    var enumerableParam = Expression.Parameter(enumerableType, "ExtractedCollection");
    var getEnumeratorFunc = Expression.Call(enumerableParam, enumerableType.GetMethod("GetEnumerator"));
    var getEnumeratorLambda = Expression.Lambda(getEnumeratorFunc, enumerableParam);
    var t1 = Expression.Assign(listVariable, Expression.Invoke(getListLambda, typeParam));
    var t2 = Expression.Assign(enumerator, Expression.Invoke(getEnumeratorLambda, listVariable));

    var @break = Expression.Label();
    var funcBlock = Expression.Block(
        new ParameterExpression[] { listVariable, enumerator},
   t1,
   t2,
    Expression.Loop(
        Expression.IfThenElse(
            Expression.NotEqual(Expression.Call(enumerator,typeof(IEnumerator).GetMethod("MoveNext")),Expression.Constant(false)),
                                Expression.Invoke(enumerableExpressions[0],Expression.Property(enumerator, "Current")),
                      Expression.Break(@break))
            , @break), typeParam);

    Expression<Action<TestObject>> lm = Expression.Lambda<Action<TestObject>>(funcBlock,recordExpression);
    var d = lm.Compile(); **//this is exceptioning with " variable 'Input' of type 'TestObject' referenced from scope '', but it is not defined**

我在你的问题中间迷路了（如果我解释错误，请告诉我，我会重新开始），但我认为这就是你所追求的：

public static Expression ForEach(Expression collection, ParameterExpression loopVar, Expression loopContent)
{
    var elementType = loopVar.Type;
    var enumerableType = typeof(IEnumerable<>).MakeGenericType(elementType);
    var enumeratorType = typeof(IEnumerator<>).MakeGenericType(elementType);
    var enumeratorVar = Expression.Variable(enumeratorType, "enumerator");
    var getEnumeratorCall = Expression.Call(collection, enumerableType.GetMethod("GetEnumerator"));
    var enumeratorAssign = Expression.Assign(enumeratorVar, getEnumeratorCall);
    // The MoveNext method's actually on IEnumerator, not IEnumerator<T>
    var moveNextCall = Expression.Call(enumeratorVar, typeof(IEnumerator).GetMethod("MoveNext"));
    var breakLabel = Expression.Label("LoopBreak");
    var loop = Expression.Block(new[] { enumeratorVar },
        enumeratorAssign,
        Expression.Loop(
            Expression.IfThenElse(
                Expression.Equal(moveNextCall, Expression.Constant(true)),
                Expression.Block(new[] { loopVar },
                    Expression.Assign(loopVar, Expression.Property(enumeratorVar, "Current")),
                    loopContent
                ),
                Expression.Break(breakLabel)
            ),
        breakLabel)
    );
    return loop;
}

要

使用它，您需要提供一个要迭代的集合、一个要替换到循环主体中的表达式，以及一个由循环体表达式使用的 ParameterExpression，该表达式将在每次循环迭代时分配给循环变量。

我认为有时例子胜于雄辩......

var collection = Expression.Parameter(typeof(List<string>), "collection");
var loopVar = Expression.Parameter(typeof(string), "loopVar");
var loopBody = Expression.Call(typeof(Console).GetMethod("WriteLine", new[] { typeof(string) }), loopVar);
var loop = ForEach(collection, loopVar, loopBody);
var compiled = Expression.Lambda<Action<List<string>>>(loop, collection).Compile();
compiled(new List<string>() { "a", "b", "c" });

编辑：正如Jeroem Mostert在评论中正确指出的那样，这并不完全反映foreach循环的"真实"行为：这将确保它释放枚举器。（它还会为每次迭代创建一个循环变量的新实例，但这对表达式没有意义）。如果您有足够的动力，实施这一点只是转动手柄的问题！

---

对于在家观看的人，我有一种类似的方法来生成"for"循环：

public static Expression For(ParameterExpression loopVar, Expression initValue, Expression condition, Expression increment, Expression loopContent)
{
    var initAssign = Expression.Assign(loopVar, initValue);
    var breakLabel = Expression.Label("LoopBreak");
    var loop = Expression.Block(new[] { loopVar },
        initAssign,
        Expression.Loop(
            Expression.IfThenElse(
                condition,
                Expression.Block(
                    loopContent,
                    increment
                ),
                Expression.Break(breakLabel)
            ),
        breakLabel)
    );
    return loop;
}

这等效于以下语句，其中伪变量与上述方法中的表达式匹配：

for (loopVar = initValue; condition; increment)
{
    loopContent
}

同样，loopContent、condition 和 increment 是使用 loopVar 的表达式，并且在每次迭代时都会分配 loopVar。

relatively_random的解决方案很棒，但foreach处理其他几种情况。检查这些指向 SharpLab 的链接，以验证每个链接中生成的内容：

• 当枚举对象是IEnumerable<T>时，它会在调用Dispose()之前检查枚举器是否null。
• 当枚举对象不是接口时，枚举器将更改为由 GetEnumerator() 返回的类型。枚举器在调用 Dispose() 之前强制转换为 IDisposable。
• 当枚举器未实现IDisposable时，as用于检查枚举器是否实现IDisposable。(???)
• 当枚举器是值类型时，null检查将消失。
• 当枚举器是值类型并且不实现IDisposable时，try/finally消失。

使用 GetEnumerator() 返回的类型非常重要，因此值类型枚举器不会被装箱。System.Collections.Generic中的所有集合都具有值类型枚举器，因为对其方法的调用不是虚拟的，因此性能要好得多。

将所有内容放在一起会产生以下代码：

static partial class ExpressionEx
{
    public static Expression ForEach<TSource>(Expression enumerable, Expression loopContent)
    {
        var enumerableType = enumerable.Type;
        var getEnumerator = enumerableType.GetMethod("GetEnumerator");
        if (getEnumerator is null)
            getEnumerator = typeof(IEnumerable<>).MakeGenericType(typeof(TSource)).GetMethod("GetEnumerator");
        var enumeratorType = getEnumerator.ReturnType;
        var enumerator = Expression.Variable(enumeratorType, "enumerator");
        return Expression.Block(new[] { enumerator },
            Expression.Assign(enumerator, Expression.Call(enumerable, getEnumerator)),
            EnumerationLoop(enumerator, loopContent));
    }
    public static Expression ForEach<TSource>(Expression enumerable, ParameterExpression loopVar, Expression loopContent)
    {
        var enumerableType = enumerable.Type;
        var getEnumerator = enumerableType.GetMethod("GetEnumerator");
        if (getEnumerator is null)
            getEnumerator = typeof(IEnumerable<>).MakeGenericType(typeof(TSource)).GetMethod("GetEnumerator");
        var enumeratorType = getEnumerator.ReturnType;
        var enumerator = Expression.Variable(enumeratorType, "enumerator");
        return Expression.Block(new[] { enumerator },
            Expression.Assign(enumerator, Expression.Call(enumerable, getEnumerator)),
            EnumerationLoop(enumerator,
                Expression.Block(new[] { loopVar },
                    Expression.Assign(loopVar, Expression.Property(enumerator, "Current")),
                    loopContent)));
    }
    static Expression EnumerationLoop(ParameterExpression enumerator, Expression loopContent)
    {
        var loop = While(
            Expression.Call(enumerator, typeof(IEnumerator).GetMethod("MoveNext")),
            loopContent);
        var enumeratorType = enumerator.Type;
        if (typeof(IDisposable).IsAssignableFrom(enumeratorType))
            return Using(enumerator, loop);
        if (!enumeratorType.IsValueType)
        {
            var disposable = Expression.Variable(typeof(IDisposable), "disposable");
            return Expression.TryFinally(
                loop,
                Expression.Block(new[] { disposable },
                    Expression.Assign(disposable, Expression.TypeAs(enumerator, typeof(IDisposable))),
                    Expression.IfThen(
                        Expression.NotEqual(disposable, Expression.Constant(null)),
                        Expression.Call(disposable, typeof(IDisposable).GetMethod("Dispose")))));
        }
        return loop;
    }
    public static Expression Using(ParameterExpression variable, Expression content)
    {
        var variableType = variable.Type;
        if (!typeof(IDisposable).IsAssignableFrom(variableType))
            throw new Exception($"'{variableType.FullName}': type used in a using statement must be implicitly convertible to 'System.IDisposable'");
        var getMethod = typeof(IDisposable).GetMethod("Dispose");
        if (variableType.IsValueType)
        {
            return Expression.TryFinally(
                content,
                Expression.Call(Expression.Convert(variable, typeof(IDisposable)), getMethod));
        }
        if (variableType.IsInterface)
        {
            return Expression.TryFinally(
                content,
                Expression.IfThen(
                    Expression.NotEqual(variable, Expression.Constant(null)),
                    Expression.Call(variable, getMethod)));
        }
        return Expression.TryFinally(
            content,
            Expression.IfThen(
                Expression.NotEqual(variable, Expression.Constant(null)),
                Expression.Call(Expression.Convert(variable, typeof(IDisposable)), getMethod)));
    }
    public static Expression While(Expression loopCondition, Expression loopContent)
    {
        var breakLabel = Expression.Label();
        return Expression.Loop(
            Expression.IfThenElse(
                loopCondition,
                loopContent,
                Expression.Break(breakLabel)),
            breakLabel);
    }
}

不带loopVar的ForEach对于在不获取项目的情况下进行枚举很有用。Count()实现就是这种情况。

编辑：NetFabric.Reflection NuGet包中提供了更新和测试的版本。检查其存储库以获取源代码。

以下是 canton7 优秀解决方案的略微扩展版本，考虑到有关处置枚举器的备注：

public static Expression ForEach(Expression enumerable, ParameterExpression loopVar, Expression loopContent)
{
    var elementType = loopVar.Type;
    var enumerableType = typeof(IEnumerable<>).MakeGenericType(elementType);
    var enumeratorType = typeof(IEnumerator<>).MakeGenericType(elementType);
    var enumeratorVar = Expression.Variable(enumeratorType, "enumerator");
    var getEnumeratorCall = Expression.Call(enumerable, enumerableType.GetMethod("GetEnumerator"));
    var enumeratorAssign = Expression.Assign(enumeratorVar, getEnumeratorCall);
    var enumeratorDispose = Expression.Call(enumeratorVar, typeof(IDisposable).GetMethod("Dispose"));
    // The MoveNext method's actually on IEnumerator, not IEnumerator<T>
    var moveNextCall = Expression.Call(enumeratorVar, typeof(IEnumerator).GetMethod("MoveNext"));
    var breakLabel = Expression.Label("LoopBreak");
    var trueConstant = Expression.Constant(true);
    var loop =
        Expression.Loop(
            Expression.IfThenElse(
                Expression.Equal(moveNextCall, trueConstant),
                Expression.Block(
                    new[] { loopVar },
                    Expression.Assign(loopVar, Expression.Property(enumeratorVar, "Current")),
                    loopContent),
                Expression.Break(breakLabel)),
            breakLabel);
    var tryFinally =
        Expression.TryFinally(
            loop,
            enumeratorDispose);
    var body =
        Expression.Block(
            new[] { enumeratorVar },
            enumeratorAssign,
            tryFinally);
    return body;
}