Question

我收到以下错误

参数＆＃39; p＆＃39;未绑定在指定的LINQ to Entities中查询表达式。

我理解这个问题（同一个ParameterExpression的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但没有运气的解决方案。

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;
    }
}

它根据许多属性为实体查找生成表达式。

它适用于单个表达式，只有在传递多个时才会出现错误。

这样称呼：

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

它会生成与我类似的内容，执行以下操作：

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

更换e.Name＆amp;＆amp; e。年龄为ConstantExpression

你可以在上面的方法中看到我抓住所有独特的参数并将它们存储在顶部的parameters中，然后在整个过程中使用相同的变量。这是开始，但后来我需要替换实例作为Expression<Func<TEntity, bool>>数组传入的每个params中的参数，这就是我失败的地方。

我已尝试枚举表达式并使用传递参数的.Update()方法

我还尝试使用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

你真的很亲密。我没有看到你parameters变量的意思。按名称对它们进行分组是一个错误。为什么不直接从表达式传递参数？然后在必要时访问。您的访客代码很好。

    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

1 个答案: