Modifying Lambda Expression

问题:

I'm developing an application with NHibernate 3.0. I have developed a Repository hat accept a expression to do some filter with QueryOver. My method is something like this:

public IEnumerable<T> FindAll(Expression<Func<T, bool>> filter) {
   return Session.QueryOver<T>().Where(filter).List();
}

It works fine. So, I have a Service layer as well, and My methods in this services accepts primitives types, like this:

public IEnumerable<Product> GetProducts(string name, int? stock, int? reserved) {

  // how init the expression ?    
  Expression<Func<Product, bool>> expression = ???;

  if (!string.IsNullOrEmpty(name)) {
     //add AND condition for name field in expression
  }  
  if (stock.HasValue) {
     //add AND condition for stock field in expression
  }
  if (reserved.HasValue) {
     //add AND condition for reserved field in expression
  }

  return _repository.FindAll(expression);
}

My doubts are:

Is it possible ? Ta add some conditions when necessary (when my parameters has value) ?

Thanks

/// my edits

public ActionResult Index(ProductFilter filter) {
   if (!string.IsNullOrEmpty(filter.Name) {
      return View(_service.GetProductsByName(filter.Name))
   }

   // others  conditions
}

/// Almost a solution

Expression<Func<Product, bool>> filter = x => true;

if (!string.IsNullOrEmpty(name))
    filter = x => filter.Compile().Invoke(x) && x.Name == name;

if (stock.HasValue) 
    filter = x => filter.Compile().Invoke(x) && x.Stock == stock.Value;

if (reserved.HasValue)
    filter = x => filter.Compile().Invoke(x) && x.Reserved == reserved.Value;

return _repository.FindAll(filter);

回答1:

Here is a way to do this. I am not going to editorialize on what you are doing - it looks like query by example, which is almost always problematic. It is as the others here have best avoided. The expression thing is interesting though - so I thought it was worth a crack at it.

class MyClass
{
     public string Name { get; set; }
     public bool Hero { get; set; }
     public int Age { get; set; }
}

And we want to query it like this:

   string name = null;
   int? age = 18;
   Expression<Func<MyClass, bool>> myExpr = 
      x => (string.IsNullOrEmpty(name) || x.Name == name) && 
           (!age.HasValue || x.Age > (age ?? 0));
   myExpr = myExpr.RemoveCloture(); // this line here - removes the cloture - 
               // and replaces it with constant values - and shortcuts 
               // boolean evaluations that are no longer necessary.
               // in effect this expression now becomes :
               // x => x.Age > 18
   bool result = myExpr.Compile()(
      new MyClass {Name = "Rondon", Hero = true, Age = 92});

So all you have to do is write RemoveCloture(); - not a problem.

// using System;
// using System.Linq.Expressions;

public static class ClotureRemover
{

#region Public Methods

public static Expression<TExpressionType> RemoveCloture<TExpressionType>(
    this Expression<TExpressionType> e)
{
    var converter = new RemoveClotureVisitor();
    var newBody = converter.Visit(e.Body);
    return Expression.Lambda<TExpressionType>(newBody, e.Parameters);
}

#endregion

private class RemoveClotureVisitor : ExpressionVisitor
{


    public RemoveClotureVisitor()
    {
    }


    public override Expression Visit(Expression node)
    {
        if (!RequiresParameterVisitor.RequiresParameter(node))
        {
            Expression<Func<object>> funct = () => new object();
            funct = Expression.Lambda<Func<object>>(Expression.Convert(node, typeof(object)), funct.Parameters);
            object res = funct.Compile()();
            return ConstantExpression.Constant(res, node.Type);
        }
        return base.Visit(node);
    }


    protected override Expression VisitBinary(BinaryExpression node)
    {
        if ((node.NodeType == ExpressionType.AndAlso) || (node.NodeType == ExpressionType.OrElse))
        {
            Expression newLeft = Visit(node.Left);
            Expression newRight = Visit(node.Right);

            bool isOr = (node.NodeType == ExpressionType.OrElse);
            bool value;
            if (IsBoolConst(newLeft, out value))
            {
                if (value ^ isOr)
                {
                    return newRight;
                }
                else
                {
                    return newLeft;
                }
            }

            if (IsBoolConst(newRight, out value))
            {
                if (value ^ isOr)
                {
                    return newLeft;
                }
                else
                {
                    return newRight;
                }
            }
        }
        return base.VisitBinary(node);
    }

    protected override Expression VisitUnary(UnaryExpression node)
    {
        if (node.NodeType == ExpressionType.Convert || node.NodeType == ExpressionType.ConvertChecked)
        {
            Expression newOpperand = Visit(node.Operand);
            if (newOpperand.Type == node.Type)
            {
                return newOpperand;
            }
        }
        return base.VisitUnary(node);
    }

    private static bool IsBoolConst(Expression node, out bool value)
    {
        ConstantExpression asConst = node as ConstantExpression;
        if (asConst != null)
        {
            if (asConst.Type == typeof(bool))
            {
                value = (bool)asConst.Value;
                return true;
            }
        }
        value = false;
        return false;
    }
}

private class RequiresParameterVisitor : ExpressionVisitor
{
    protected RequiresParameterVisitor()
    {
        result = false;
    }

    public static bool RequiresParameter(Expression node)
    {
        RequiresParameterVisitor visitor = new RequiresParameterVisitor();
        visitor.Visit(node);
        return visitor.result;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        result = true;
        return base.VisitParameter(node);
    }

    internal bool result;
}

}

回答2:

First, I'd solve your problem by avoiding it in the first place. I'd have different methods for this.

public IEnumerable<Product> GetProductsByName(string name)
public IEnumerable<Product> GetProudctsByNameAndStock(string name, int stock)
public IEnumerable<Product> GetProductsByNameAndReserved(
    string name,
    int reserved
)
public IEnumerable<Product> GetProducts(string name, int stock, int reserved)

These all have trivially easy implementations in terms of a lambda expression. For example:

public IEnumerable<Product> GetProductsByName(string name) {
    return GetProductsByExpression(p => p.Name == name);
}

private IEnumerable<Product> GetProductsByExpression(
    Expression<Func<Product, bool>> expression
) {
    return _repository.FindAll(expression);
}

etc.

Is it possible ? Ta add some conditions when necessary (when my parameters has value) ?

Second, yes what you want to do is possible but it's not the way I'd solve the problem.

回答3:

Your repository method definition suggests that you see FindAll as something that you pass criteria in and get a completed result back. Why not instead just have the result be of type IQueryable and return Session.QueryOver?

Your service layer would then do something like this, chaining together the "wheres":

var query = _repository.FindAll();
if (!string.IsNullOrEmpty(name))
  query = query.Where(x => x.Name == name);
if (stock.HasValue)
  query = query.Where(x => x.Stock == stock);
etc...


return query.ToList();

回答4:

So here is how you could actually and lambdas together - it borrows most of it's code from this awesome answer from desco that deserves an up-vote.

public static class AddExpressions
{
   public static Expression<Func<TFrom, TTo>> AndLambdas<TFrom, TTo>(this Expression<Func<TFrom, TTo>> first, Expression<Func<TFrom, TTo>> second)
   {    
     ParameterExpression paramToUse = first.Parameters[0];
     Expression bodyLeft = first.Body;
     ConversionVisitor visitor = new ConversionVisitor(paramToUse, second.Parameters[0]);
     Expression bodyRight = visitor.Visit(second.Body);
     return Expression.Lambda<Func<TFrom, TTo>>(Expression.MakeBinary(ExpressionType.AndAlso, bodyLeft, bodyRight), first.Parameters);
   }

class ConversionVisitor : ExpressionVisitor
{
    private readonly ParameterExpression newParameter;
    private readonly ParameterExpression oldParameter;

    public ConversionVisitor(ParameterExpression newParameter, ParameterExpression oldParameter)
    {
        this.newParameter = newParameter;
        this.oldParameter = oldParameter;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return newParameter; // replace all old param references with new ones
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Expression != oldParameter) // if instance is not old parameter - do nothing
            return base.VisitMember(node);

        var newObj = Visit(node.Expression);
        var newMember = newParameter.Type.GetMember(node.Member.Name).First();
        return Expression.MakeMemberAccess(newObj, newMember);
    }
}

}

Then calling the code is quite simple ....

    class MyClass
    {
        public string Name { get; set; }
        public bool Hero { get; set; }
        public int Age { get; set; }

    }

...

 Expression<Func<MyClass, bool>> expression1 = x => x.Age > (age ?? 0);
 Expression<Func<MyClass, bool>> expression2 = x => x.Name == name;

 expression1 = expression1.AndLambdas(expression2);
 result = expression1.Compile()(new MyClass { 
            Name = "Rondon", 
            Hero = true, 
            Age = 92 });