c# - 组合表达式(表达式> 与表达式>)

Question

我有两个表达式:

Expression<Func<T1, T2>> 
Expression<Func<T2, bool>> 

我想将这些结合起来并获得一个类型为 Expression > 的新表达式，以便它可以在 Entity Framework LINQ 中使用。

我在 Expression.Invoke() 的帮助下将它们结合起来，但它不起作用。

//Extensinon method
public static Expression<Func<T1, bool>> Compose<T1, T2>(this Expression<Func<T1, T2>> convertExpr, Expression<Func<T2, bool>> predicate) 
    => Expression.Lambda<Func<T1, bool>>(Expression.Invoke(predicate, convertExpr.Body), convertExpr.Parameters.First());
...
Expression<Func<One, Two>> convert;
Expression<Func<Two, bool>> predicate;
Expression<Func<One, bool>> filter=convert.Compose(predicate);

// Works fine
List<One> lst;
lst.AsQueryable().Where(filter);

DbSet<One> src;
// In next line throws runtime NotSupportedException: The LINQ expression node type 'Invoke' is not supported in LINQ to Entities
src.Where(filter);

更新

例如:

public class Book
{ 
  public int Id {get; protected set;}
  public string Name {get; protected set;}
  public virtual Genre {get; protected set;}
}

public class Genre
{
  public int Id {get; protected set;}
  public string Name {get; protected set;}
}
...
Expression<Func<Book, Genre>> convert = b=>b.Genre;
Expression<Func<Genre, bool>> predicate = g=>g.Name=="fantasy";
// Need: b=>b.Genre=="fantasy"
Expression<Func<Genre, bool>> filter=convert.Compose(predicate);

// Works fine
List<Book> lst;
lst.AsQueryable().Where(filter);

DbSet<Book> books;
// In next line throws runtime NotSupportedException: The LINQ expression node type 'Invoke' is not supported in LINQ to Entities
books.Where(filter);

Answer 1

使用通用的 ExpressionVisitor 将一个 Expression 替换为另一个，我的标准 Compose 函数(我认为以更常见的数学顺序) , 将一个 LambdaExpression 的 Body 替换为另一个中的参数:

public static class ExpressionExt {
    // Compose: (y => f(y)).Compose(x => g(x)) -> x => f(g(x))
    /// <summary>
    /// Composes two LambdaExpression into a new LambdaExpression
    /// </summary>
    /// <param name="Tpg">Type of parameter to gFn, and type of parameter to result lambda.</param>
    /// <param name="Tpf">Type of result of gFn and type of parameter to fFn.</param>
    /// <param name="TRes">Type of result of fFn and type of result of result lambda.</param>
    /// <param name="fFn">The outer LambdaExpression.</param>
    /// <param name="gFn">The inner LambdaExpression.</param>
    /// <returns>LambdaExpression representing outer composed with inner</returns>
    public static Expression<Func<Tpg, TRes>> Compose<Tpg, Tpf, TRes>(this Expression<Func<Tpf, TRes>> fFn, Expression<Func<Tpg, Tpf>> gFn) =>
        Expression.Lambda<Func<Tpg, TRes>>(fFn.Body.Replace(fFn.Parameters[0], gFn.Body), gFn.Parameters[0]);

    /// <summary>
    /// Replaces an Expression (reference Equals) with another Expression
    /// </summary>
    /// <param name="orig">The original Expression.</param>
    /// <param name="from">The from Expression.</param>
    /// <param name="to">The to Expression.</param>
    /// <returns>Expression with all occurrences of from replaced with to</returns>
    public static Expression Replace(this Expression orig, Expression from, Expression to) => new ReplaceVisitor(from, to).Visit(orig);
}

/// <summary>
/// ExpressionVisitor to replace an Expression (that is Equals) with another Expression.
/// </summary>
public class ReplaceVisitor : ExpressionVisitor {
    readonly Expression from;
    readonly Expression to;

    public ReplaceVisitor(Expression from, Expression to) {
        this.from = from;
        this.to = to;
    }

    public override Expression Visit(Expression node) => node == from ? to : base.Visit(node);
}

有了这个，你的例子就很简单了:

Expression<Func<One, Two>> convert = p1 => new Two(p1);
Expression<Func<Two, bool>> predicate = p2 => p2 == new Two();
Expression<Func<One, bool>> filter = predicate.Compose(convert);

但是，尤其是对于 EF 表达式，最好使用我的 Invoke 替代品，它处理 null 传播，在以下情况下您的参数可能是 null。它还使用与上面相同的 Replace ExpressionVisitor:

public static class ExpressionExt2 {   
    public static Expression PropagateNull(this Expression orig) => new NullVisitor().Visit(orig);

    // Apply: (x => f).Apply(args)
    /// <summary>
    /// Substitutes an array of Expression args for the parameters of a lambda, returning a new Expression
    /// </summary>
    /// <param name="e">The original LambdaExpression to "call".</param>
    /// <param name="args">The Expression[] of values to substitute for the parameters of e.</param>
    /// <returns>Expression representing e.Body with args substituted in</returns>
    public static Expression Apply(this LambdaExpression e, params Expression[] args) {
        var b = e.Body;

        foreach (var pa in e.Parameters.Zip(args, (p, a) => (p, a)))
            b = b.Replace(pa.p, pa.a);

        return b.PropagateNull();
    }
}

/// <summary>
/// ExpressionVisitor to replace a null.member Expression with a null
/// </summary>
public class NullVisitor : System.Linq.Expressions.ExpressionVisitor {
    public override Expression Visit(Expression node) {
        if (node is MemberExpression nme && nme.Expression is ConstantExpression nce && nce.Value == null)
            return Expression.Constant(null, nce.Type.GetMember(nme.Member.Name).Single().GetMemberType());
        else
            return base.Visit(node);
    }
}

public static class MeberInfoExt {
    public static Type GetMemberType(this MemberInfo member) {
        switch (member) {
            case FieldInfo mfi:
                return mfi.FieldType;
            case PropertyInfo mpi:
                return mpi.PropertyType;
            case EventInfo mei:
                return mei.EventHandlerType;
            default:
                throw new ArgumentException("MemberInfo must be if type FieldInfo, PropertyInfo or EventInfo", nameof(member));
        }
    }    
}

鉴于 Apply，您的 Compose 很简单:

public static Expression<Func<T1, bool>> Compose<T1, T2>(this Expression<Func<T1, T2>> convertExpr, Expression<Func<T2, bool>> predicate)
    => Expression.Lambda<Func<T1, bool>>(predicate.Apply(convertExpr.Body), convertExpr.Parameters.First());