我觉得这样挺好的，不是吗?

Func<T, bool> expr1 = (x => x.Att1 == "a");
Func<T, bool> expr2 = (x => x.Att2 == "b");
Func<T, bool> expr1ANDexpr2 = (x => expr1(x) && expr2(x));
Func<T, bool> expr1ORexpr2 = (x => expr1(x) || expr2(x));
Func<T, bool> NOTexpr1 = (x => !expr1(x));

你可以用表达式。和also / OrElse等结合逻辑表达式，但问题是参数;你在expr1和expr2中使用相同的参数表达式吗?如果是这样，那么:

var body = Expression.AndAlso(expr1.Body, expr2.Body);
var lambda = Expression.Lambda<Func<T,bool>>(body, expr1.Parameters[0]);

这也适用于对单个操作求反:

static Expression<Func<T, bool>> Not<T>(
    this Expression<Func<T, bool>> expr)
{
    return Expression.Lambda<Func<T, bool>>(
        Expression.Not(expr.Body), expr.Parameters[0]);
}

否则，取决于LINQ提供程序，你可以将它们与Invoke结合使用:

// OrElse is very similar...
static Expression<Func<T, bool>> AndAlso<T>(
    this Expression<Func<T, bool>> left,
    Expression<Func<T, bool>> right)
{
    var param = Expression.Parameter(typeof(T), "x");
    var body = Expression.AndAlso(
            Expression.Invoke(left, param),
            Expression.Invoke(right, param)
        );
    var lambda = Expression.Lambda<Func<T, bool>>(body, param);
    return lambda;
}

在某个地方，我有一些代码重写了一个表达式树，替换节点以消除对Invoke的需求，但它相当长(我不记得我把它放在哪里了……)

选择最简单路线的通用版本:

static Expression<Func<T, bool>> AndAlso<T>(
    this Expression<Func<T, bool>> expr1,
    Expression<Func<T, bool>> expr2)
{
    // need to detect whether they use the same
    // parameter instance; if not, they need fixing
    ParameterExpression param = expr1.Parameters[0];
    if (ReferenceEquals(param, expr2.Parameters[0]))
    {
        // simple version
        return Expression.Lambda<Func<T, bool>>(
            Expression.AndAlso(expr1.Body, expr2.Body), param);
    }
    // otherwise, keep expr1 "as is" and invoke expr2
    return Expression.Lambda<Func<T, bool>>(
        Expression.AndAlso(
            expr1.Body,
            Expression.Invoke(expr2, param)), param);
}

从。net 4.0开始，有了ExpressionVisitor类，它允许你构建EF安全的表达式。

    public static Expression<Func<T, bool>> AndAlso<T>(
        this Expression<Func<T, bool>> expr1,
        Expression<Func<T, bool>> expr2)
    {
        var parameter = Expression.Parameter(typeof (T));

        var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
        var left = leftVisitor.Visit(expr1.Body);

        var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);
        var right = rightVisitor.Visit(expr2.Body);

        return Expression.Lambda<Func<T, bool>>(
            Expression.AndAlso(left, right), parameter);
    }



    private class ReplaceExpressionVisitor
        : ExpressionVisitor
    {
        private readonly Expression _oldValue;
        private readonly Expression _newValue;

        public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
        {
            _oldValue = oldValue;
            _newValue = newValue;
        }

        public override Expression Visit(Expression node)
        {
            if (node == _oldValue)
                return _newValue;
            return base.Visit(node);
        }
    }

我在这里组合了一些漂亮的答案，以便能够轻松地支持更多的表达式操作符。

这是基于@Dejan的回答，但现在添加OR也很容易。我选择不将Combine函数设为公共的，但是您可以这样做，以便更加灵活。

public static class ExpressionExtensions
{
    public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> leftExpression,
        Expression<Func<T, bool>> rightExpression) =>
        Combine(leftExpression, rightExpression, Expression.AndAlso);

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> leftExpression,
        Expression<Func<T, bool>> rightExpression) =>
        Combine(leftExpression, rightExpression, Expression.Or);

    public static Expression<Func<T, bool>> Combine<T>(Expression<Func<T, bool>> leftExpression, Expression<Func<T, bool>> rightExpression, Func<Expression, Expression, BinaryExpression> combineOperator)
    {
        var leftParameter = leftExpression.Parameters[0];
        var rightParameter = rightExpression.Parameters[0];

        var visitor = new ReplaceParameterVisitor(rightParameter, leftParameter);

        var leftBody = leftExpression.Body;
        var rightBody = visitor.Visit(rightExpression.Body);

        return Expression.Lambda<Func<T, bool>>(combineOperator(leftBody, rightBody), leftParameter);
    }

    private class ReplaceParameterVisitor : ExpressionVisitor
    {
        private readonly ParameterExpression _oldParameter;
        private readonly ParameterExpression _newParameter;

        public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
        {
            _oldParameter = oldParameter;
            _newParameter = newParameter;
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return ReferenceEquals(node, _oldParameter) ? _newParameter : base.VisitParameter(node);
        }
    }
}

用法没有改变，仍然是这样的:

Expression<Func<Result, bool>> noFilterExpression = item => filters == null;

Expression<Func<Result, bool>> laptopFilterExpression = item => item.x == ...
Expression<Func<Result, bool>> dateFilterExpression = item => item.y == ...

var combinedFilterExpression = noFilterExpression.Or(laptopFilterExpression.AndAlso(dateFilterExpression));
    
efQuery.Where(combinedFilterExpression);

(这是一个基于我的实际代码的示例，但将其视为伪代码)

如果您的提供程序不支持Invoke，而您需要组合两个表达式，则可以使用ExpressionVisitor将第二个表达式中的参数替换为第一个表达式中的参数。

class ParameterUpdateVisitor : ExpressionVisitor
{
    private ParameterExpression _oldParameter;
    private ParameterExpression _newParameter;

    public ParameterUpdateVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
    {
        _oldParameter = oldParameter;
        _newParameter = newParameter;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (object.ReferenceEquals(node, _oldParameter))
            return _newParameter;

        return base.VisitParameter(node);
    }
}

static Expression<Func<T, bool>> UpdateParameter<T>(
    Expression<Func<T, bool>> expr,
    ParameterExpression newParameter)
{
    var visitor = new ParameterUpdateVisitor(expr.Parameters[0], newParameter);
    var body = visitor.Visit(expr.Body);

    return Expression.Lambda<Func<T, bool>>(body, newParameter);
}

[TestMethod]
public void ExpressionText()
{
    string text = "test";

    Expression<Func<Coco, bool>> expr1 = p => p.Item1.Contains(text);
    Expression<Func<Coco, bool>> expr2 = q => q.Item2.Contains(text);
    Expression<Func<Coco, bool>> expr3 = UpdateParameter(expr2, expr1.Parameters[0]);

    var expr4 = Expression.Lambda<Func<Recording, bool>>(
        Expression.OrElse(expr1.Body, expr3.Body), expr1.Parameters[0]);

    var func = expr4.Compile();

    Assert.IsTrue(func(new Coco { Item1 = "caca", Item2 = "test pipi" }));
}

我需要实现相同的结果，但是使用一些更通用的东西(因为不知道类型)。多亏了marc的回答，我终于明白了我想要达到的目标:

    public static LambdaExpression CombineOr(Type sourceType, LambdaExpression exp, LambdaExpression newExp) 
    {
        var parameter = Expression.Parameter(sourceType);

        var leftVisitor = new ReplaceExpressionVisitor(exp.Parameters[0], parameter);
        var left = leftVisitor.Visit(exp.Body);

        var rightVisitor = new ReplaceExpressionVisitor(newExp.Parameters[0], parameter);
        var right = rightVisitor.Visit(newExp.Body);

        var delegateType = typeof(Func<,>).MakeGenericType(sourceType, typeof(bool));
        return Expression.Lambda(delegateType, Expression.Or(left, right), parameter);
    }

using System;
using System.Linq.Expressions;

namespace Extensions
{


    public class Example
    {
        //How to use it
        public static void Main()
        {
            Expression<Func<string, bool>> expression1 = exp => true;
            Expression<Func<string, bool>> expression2 = exp => false;

            Expression<Func<string, bool>> expression3 = ExpressionExtensions.AndAlso(expression1, expression2);
            Expression<Func<string, bool>> expression4 = ExpressionExtensions.OrElse(expression1, expression2);

            Expression<Func<string, bool>> expression = ExpressionExtensions.AndAlso(expression3, expression4);
        }
    }


    public static class ExpressionExtensions
    {
        public static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            ParameterExpression parameter1 = expr1.Parameters[0];
            var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
            var body2WithParam1 = visitor.Visit(expr2.Body);
            return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, body2WithParam1), parameter1);
        }

        public static Expression<Func<T, bool>> OrElse<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            ParameterExpression parameter1 = expr1.Parameters[0];
            var visitor = new ReplaceParameterVisitor(expr2.Parameters[0], parameter1);
            var body2WithParam1 = visitor.Visit(expr2.Body);
            return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, body2WithParam1), parameter1);
        }

        private class ReplaceParameterVisitor : ExpressionVisitor
        {
            private readonly ParameterExpression _oldParameter;
            private readonly ParameterExpression _newParameter;

            public ReplaceParameterVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
            {
                _oldParameter = oldParameter;
                _newParameter = newParameter;
            }

            protected override Expression VisitParameter(ParameterExpression node)
            {
                if (ReferenceEquals(node, _oldParameter))
                    return _newParameter;

                return base.VisitParameter(node);
            }
        }
    }
}