组合多个表达式树

Question

我收到以下错误

The parameter 'p' was not bound in the specified LINQ to Entities query expression.

我理解这个问题（ParameterExpression 的同一个实例应该与树中的所有表达式一起使用）并尝试使用我在网上找到的解决方案，但没有成功。

这是我的方法

private void SeedEntity<TEntity>(DatabaseContext context, ref TEntity entity, params Expression<Func<TEntity, object>>[] identifierExpressions) where TEntity : class
{
    Expression<Func<TEntity, bool>> allExpresions = null;

    var parameters = identifierExpressions.SelectMany(x => x.Parameters).GroupBy(x => x.Name).Select(p => p.First()).ToList();

    foreach (Expression<Func<TEntity, object>> identifierExpression in identifierExpressions)
    {
        Func<TEntity, object> vv = identifierExpression.Compile();
        object constant = vv(entity);

        ConstantExpression constExp = Expression.Constant(constant, typeof(object));
        BinaryExpression equalExpression1 = Expression.Equal(identifierExpression.Body, constExp);
        Expression<Func<TEntity, bool>> equalExpression2 = Expression.Lambda<Func<TEntity, bool>>(equalExpression1, parameters);

        if (allExpresions == null)
        {
            allExpresions = equalExpression2;
        }
        else
        {
            BinaryExpression bin = Expression.And(allExpresions.Body, equalExpression2.Body);
            allExpresions = Expression.Lambda<Func<TEntity, bool>>(bin, parameters);
        }
    }

    TEntity existingEntity = null;
    if (allExpresions != null)
    {
        existingEntity = context.Set<TEntity>().FirstOrDefault(allExpresions);
    }

    if (existingEntity == null)
    {
        context.Set<TEntity>().Add(entity);
    }
    else
    {
        entity = existingEntity;
    }
}

它根据许多属性生成用于查找实体的表达式。

对单个表达式没问题，只在传入多个时出错

这样调用：

SeedEntity(context, ref e, p=> p.Name);//Works
SeedEntity(context, ref e, p=> p.Name, p=> p.Age);//Fails

它会生成类似于我执行以下操作的内容：

context.Set<TEntity>().FirstOrDefault(p=>p.Name == e.Name && p.Age == e.Age);

用 ConstantExpression

替换 e.Name && e.Age

你可以看到在上面的方法中我获取了所有唯一的参数并将它们存储在顶部的 parameters 中，然后使用相同的变量 throughout.This 是开始，但是我需要替换作为 params 数组传入的每个 Expression<Func<TEntity, bool>> 中的参数实例，这是我失败的地方。

我尝试枚举表达式并使用 .Update() 方法传入参数

我也尝试了使用 ExpressionVisitor

的解决方案

public class ExpressionSubstitute : ExpressionVisitor
{
    public readonly Expression from, to;
    public ExpressionSubstitute(Expression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
        if (node == from) return to;
        return base.Visit(node);
    }
}

public static class ExpressionSubstituteExtentions
{
    public static Expression<Func<TEntity, TReturnType>> RewireLambdaExpression<TEntity, TReturnType>(Expression<Func<TEntity, TReturnType>> expression, ParameterExpression newLambdaParameter)
    {
        var newExp = new ExpressionSubstitute(expression.Parameters.Single(), newLambdaParameter).Visit(expression);
        return (Expression<Func<TEntity, TReturnType>>)newExp;
    }
}

Answer 1

你真的很接近。我不明白你的 parameters 变量的意义。按名称对它们进行分组是错误的。为什么不直接传递表达式中的参数呢？必要时再去拜访。您的访客代码没问题。

    private static void SeedEntity<TEntity>(DbContext context, ref TEntity entity, params Expression<Func<TEntity, object>>[] identifierExpressions) 
        where TEntity : class
    {
        Expression<Func<TEntity, bool>> allExpresions = null;

        foreach (Expression<Func<TEntity, object>> identifierExpression in identifierExpressions)
        {
            Func<TEntity, object> vv = identifierExpression.Compile();
            object constant = vv(entity);

            ConstantExpression constExp = Expression.Constant(constant, typeof(object));
            BinaryExpression equalExpression1 = Expression.Equal(identifierExpression.Body, constExp);
            Expression<Func<TEntity, bool>> equalExpression2 = Expression.Lambda<Func<TEntity, bool>>(equalExpression1, identifierExpression.Parameters);

            if (allExpresions == null)
            {
                allExpresions = equalExpression2;
            }
            else
            {
                var visitor = new ExpressionSubstitute(allExpresions.Parameters[0], identifierExpression.Parameters[0]);
                var modifiedAll = (Expression<Func<TEntity,bool>>)visitor.Visit(allExpresions);
                BinaryExpression bin = Expression.And(modifiedAll.Body, equalExpression2.Body);
                allExpresions = Expression.Lambda<Func<TEntity, bool>>(bin, identifierExpression.Parameters);
            }
        }

        TEntity existingEntity = null;
        if (allExpresions != null)
        {
            existingEntity = context.Set<TEntity>().FirstOrDefault(allExpresions);
        }

        if (existingEntity == null)
        {
            context.Set<TEntity>().Add(entity);
        }
        else
        {
            entity = existingEntity;
        }
    }

组合多个表达式树

Combining multiple expressions trees

.net

c#

entity-framework

expression-trees