修改Lambda表达式 [英] Modifying Lambda Expression
问题描述
我正在使用NHibernate 3.0开发应用程序.我已经开发了一个Repository帽子，可以接受一个表达式来对QueryOver进行一些过滤.我的方法是这样的:
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();
}
工作正常.因此，我也有一个Service层，并且该服务中的My方法接受原始类型，如下所示:
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);
}
我的疑问是:
有可能吗?必要时(当我的参数有价值时)添加一些条件吗?
Is it possible ? Ta add some conditions when necessary (when my parameters has value) ?
谢谢
///我的修改
public ActionResult Index(ProductFilter filter) {
if (!string.IsNullOrEmpty(filter.Name) {
return View(_service.GetProductsByName(filter.Name))
}
// others conditions
}
///几乎是一个解决方案
/// 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);
推荐答案
这里是实现此目的的一种方法.我不会对您正在做的事情进行编辑-看起来像通过示例查询，这几乎总是有问题的.正如这里的其他人最好避免的那样.表情虽然很有趣-所以我认为值得一试.
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; }
}
我们想这样查询它:
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});
所以您要做的就是写RemoveCloture();-没问题.
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;
}
}
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