如何编译一个表达式到实际的结果呢？ [英] How to compile an Expression down to the actual result?
问题描述
我使用表达式允许开发人员指定的查询，并有ExpressionVisitor转换成表达式查询字符串构建一个API围绕Web服务调用。该请求是XML包含查询字符串的特定元素。
I am building an API around a web service call using Expressions to allow a developer to specify a query and have an ExpressionVisitor convert the Expression into the query string. The request is XML with a particular element containing a query string.
例如，我可以做这样的事情，将与银行1的银行名称检索所有支票账户或银行2：
For example, I can do something like this which will retrieve all checking accounts with a bank name of Bank 1 or Bank 2:
BANKNAME ='银行1'或BANKNAME ='银行2'。
"bankname = 'Bank 1' or bankname = 'Bank 2'"
。网络服务可以处理显著更复杂的查询，但我就这个坚持现在
The web service can handle significantly more complex queries but I'll just stick with this for now.
所以我有一个类的CheckingAccount：
So I have a class CheckingAccount:
[IntacctObject("checkingaccount")]
public class CheckingAccount : Entity
{
[IntacctElement("bankaccountid")]
public string Id { get; set; }
[IntacctElement("bankname")]
public string BankName { get; set; }
}
和一个ExpressionVisitor，其主要功能是将一个表达式转换这样的：
And an ExpressionVisitor whose primary function is to convert an expression like this:
Expression> expression = ca => ca.BankName == "Bank 1" || ca.BankName == "Bank 2"
进入查询：BANKNAME ='银行1'或BANKNAME ='银行2'
into the query: "bankname = 'Bank 1' or bankname = 'Bank 2'"
这是不那么强硬。事情开始分解的时候我介绍的局部变量：
This isn't so tough. Where things start to break down are when I introduce local variables:
var account = new CheckingAccount { BankName = "Bank 1" };
string bankName = "Bank 2";
Expression> expression = ca => ca.BankName == account.BankName || ca.BankName == bankName;
我知道如何处理一个简单的局部变量（即字符串BANKNAME =银行2）但处理一个另一种类型（无功帐户=新的CheckingAccount {BANKNAME =银行1}）复杂得多。
I know how to deal with a simple local variable (ie. string bankName = "Bank 2") but dealing with a the other type (var account = new CheckingAccount { BankName = "Bank 1" }) is much more complex.
在一天这些的端我需要弄清楚如何处理眼下的大问题。我知道有更复杂的方案，但我不那么关心这些的时刻。
At the end of the day these are the big issues that I need to figure out how to deal with right now. I know there are much more complex scenarios but I'm not so concerned with those at the moment.
下面是我的表达访问者（请注意，方法CreateFilter通用约束）：
Here is my expression visitor (please note the generic constraint on method CreateFilter):
internal class IntacctWebService30ExpressionVisitor : ExpressionVisitor
{
private readonly List _Filters = new List();
private IntacctWebServicev30SimpleQueryFilter _CurrentSimpleFilter;
private IntacctWebServicev30ComplexQueryFilter _CurrentComplexFilter;
private MemberExpression _CurrentMemberExpression;
public string CreateFilter(Expression> expression) where TEntity : Entity
{
Visit(expression);
string filter = string.Join(string.Empty, _Filters.Select(f => f.ToString()).ToArray());
return filter;
}
protected override Expression VisitBinary(BinaryExpression node)
{
switch (node.NodeType)
{
case ExpressionType.AndAlso:
case ExpressionType.OrElse:
_CurrentComplexFilter = new IntacctWebServicev30ComplexQueryFilter { ExpressionType = node.NodeType };
break;
case ExpressionType.Equal:
case ExpressionType.GreaterThan:
case ExpressionType.GreaterThanOrEqual:
case ExpressionType.LessThan:
case ExpressionType.LessThanOrEqual:
case ExpressionType.NotEqual:
_CurrentSimpleFilter = new IntacctWebServicev30SimpleQueryFilter { ExpressionType = node.NodeType };
break;
}
return base.VisitBinary(node);
}
protected override Expression VisitMember(MemberExpression node)
{
var attr = node.Member.GetAttribute();
if (attr != null)
_CurrentSimpleFilter.FieldName = attr.FieldName;
else
_CurrentMemberExpression = node;
return base.VisitMember(node);
}
protected override Expression VisitConstant(ConstantExpression node)
{
object value = Expression.Lambda>(node).Compile().Invoke();
string fieldValue = extraxtFieldValue(value, node);
_CurrentSimpleFilter.FieldValue = fieldValue;
if (_CurrentComplexFilter != null)
{
if (_CurrentComplexFilter.Left == null)
{
_CurrentComplexFilter.Left = _CurrentSimpleFilter;
}
else if (_CurrentComplexFilter.Right == null)
{
_CurrentComplexFilter.Right = _CurrentSimpleFilter;
_Filters.Add(_CurrentComplexFilter);
}
else
throw new InvalidOperationException();
}
else
{
_Filters.Add(_CurrentSimpleFilter);
}
return base.VisitConstant(node);
}
private string extraxtFieldValue(object value)
{
string fieldValue;
if (value is DateTime)
fieldValue = ((DateTime)value).ToString("yyyy-MM-dd");
else if (value is string)
fieldValue = value.ToString();
else if (value.GetType().IsEnum)
{
throw new NotImplementedException();
}
else
{
// Not sure if this is the best way to do this or not but can't figure out how
// else to get a variable value.
// If we are here then we are dealing with a property, field, or variable.
// This means we must extract the value from the object.
// In order to do this we will rely on _CurrentMemberExpression
if (_CurrentMemberExpression.Member.MemberType == MemberTypes.Property)
{
fieldValue = value.GetType().GetProperty(_CurrentMemberExpression.Member.Name).GetValue(value, null).ToString();
}
else if (_CurrentMemberExpression.Member.MemberType == MemberTypes.Field)
{
fieldValue = value.GetType().GetFields().First().GetValue(value).ToString();
}
else
{
throw new InvalidOperationException();
}
}
return fieldValue;
}
}
请让我知道，如果你想更多的详细...
Please let me know if you'd like any more detail....
感谢
推荐答案
看一看的有关从马特·沃伦就此问题文章。他为一个类，正是这一点，有一个解释，它是如何做到这一点提供了来源。它也包含在他IQToolkit 。
Have a look at an article about this very issue from Matt Warren. He provides source for a class that does exactly this, with an explanation how does it do it. It's also included in his IQToolkit.
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