今天在百度知道中看到一個問題，研究了一會便回答了：
http://zhidao.baidu.com/question/920461189016484459.html
如何使dto linq 表達式轉換到數據庫實體對象linq表達式。自己搜集了一些資料然後實戰了一下，還是可行。
自己擴展的一個方法 Cast<TInput, TToProperty>(this Expression<Func<TInput, bool>> expression)，代碼如下：
namespace System
{
public static class LambdaExpressionExtensions
{
private static Expression Parser(ParameterExpression parameter, Expression expression)
{
if (expression == null) return null;
switch (expression.NodeType)
{
//一元運算符
case ExpressionType.Negate:
case ExpressionType.NegateChecked:
case ExpressionType.Not:
case ExpressionType.Convert:
case ExpressionType.ConvertChecked:
case ExpressionType.ArrayLength:
case ExpressionType.Quote:
case ExpressionType.TypeAs:
{
var unary = expression as UnaryExpression;
var exp = Parser(parameter, unary.Operand);
return Expression.MakeUnary(expression.NodeType, exp, unary.Type, unary.Method);
}
//二元運算符
case ExpressionType.Add:
case ExpressionType.AddChecked:
case ExpressionType.Subtract:
case ExpressionType.SubtractChecked:
case ExpressionType.Multiply:
case ExpressionType.MultiplyChecked:
case ExpressionType.Divide:
case ExpressionType.Modulo:
case ExpressionType.And:
case ExpressionType.AndAlso:
case ExpressionType.Or:
case ExpressionType.OrElse:
case ExpressionType.LessThan:
case ExpressionType.LessThanOrEqual:
case ExpressionType.GreaterThan:
case ExpressionType.GreaterThanOrEqual:
case ExpressionType.Equal:
case ExpressionType.NotEqual:
case ExpressionType.Coalesce:
case ExpressionType.ArrayIndex:
case ExpressionType.RightShift:
case ExpressionType.LeftShift:
case ExpressionType.ExclusiveOr:
{
var binary = expression as BinaryExpression;
var left = Parser(parameter, binary.Left);
var right = Parser(parameter, binary.Right);
var conversion = Parser(parameter, binary.Conversion);
if (binary.NodeType == ExpressionType.Coalesce && binary.Conversion != null)
{
return Expression.Coalesce(left, right, conversion as LambdaExpression);
}
else
{
return Expression.MakeBinary(expression.NodeType, left, right, binary.IsLiftedToNull, binary.Method);
}
}
//其他
case ExpressionType.Call:
{
var call = expression as MethodCallExpression;
List<Expression> arguments = new List<Expression>();
foreach (var argument in call.Arguments)
{
arguments.Add(Parser(parameter, argument));
}
var instance = Parser(parameter, call.Object);
call = Expression.Call(instance, call.Method, arguments);
return call;
}
case ExpressionType.Lambda:
{
var Lambda = expression as LambdaExpression;
return Parser(parameter, Lambda.Body);
}
case ExpressionType.MemberAccess:
{
var memberAccess = expression as MemberExpression;
if (memberAccess.Expression == null)
{
memberAccess = Expression.MakeMemberAccess(null, memberAccess.Member);
}
else
{
var exp = Parser(parameter, memberAccess.Expression);
var member = exp.Type.GetMember(memberAccess.Member.Name).FirstOrDefault();
memberAccess = Expression.MakeMemberAccess(exp, member);
}
return memberAccess;
}
case ExpressionType.Parameter:
return parameter;
case ExpressionType.Constant:
return expression;
case ExpressionType.TypeIs:
{
var typeis = expression as TypeBinaryExpression;
var exp = Parser(parameter, typeis.Expression);
return Expression.TypeIs(exp, typeis.TypeOperand);
}
default:
throw new Exception(string.Format("Unhandled expression type: '{0}'", expression.NodeType));
}
}
public static Expression<Func<TToProperty, bool>> Cast<TInput, TToProperty>(this Expression<Func<TInput, bool>> expression)
{
var p = Expression.Parameter(typeof(TToProperty), "p");
var x = Parser(p, expression);
return Expression.Lambda<Func<TToProperty, bool>>(x, p);
}
}
}
比如有如下的 實體類對象：
public class User
{
public int Id { get; set; }
public string Name { get; set; }
}
public class UserDto
{
public int Id { get; set; }
public string Name { get; set; }
}
簡單的測試代碼：
class Program
{
static int[] array0 = new[] { 0, 1 };
static void Main1(string[] args)
{
var array1 = new[] { 0, 1 };
Expression<Func<UserDto, bool>> exp = null;
Expression<Func<User, bool>> exp2 = null;
//====exp====
//exp = u => u.Name == "張三";
//exp = u => u.Id.Equals(1);
//exp = u => u.Id.Equals(1) && u.Name == "張三";
//exp = u => u.Id.Equals(1) && u.Name == "張三" || u.Name == "張三";
//exp = u => Filter(u.Name);
//exp = u => !Filter(u.Name);
//exp = u => u.Id.Equals(1) && u.Name == "張三" && Filter(u.Name);
//exp = u => array1.Contains(u.Id);
//exp = u => array1.Contains(u.Id) || u.Name == "張三";
//exp = u => array0.Contains(u.Id);
//exp = u => u.Id > 0;
//exp = u => u.Id < 10;
//exp = u => u.Id * 2 < 10;
//exp = u => u.Id - 2 < 10;
//exp = u => u.Id + 2 < 10;
//exp = u => u.Id / 2 < 10;
//exp = u => (int)(u.Id / 2) < 10;
//exp = u => u.Name is string;
//exp = u => ((object)u.Id).ToString() == "1";
//exp = u => u.Id == default(int);
//exp = u => true;
//exp = u => Math.Abs(u.Id)==1;
exp = u =>
u.Id.Equals(1)
&& u.Name == "張三"
&& u.Id < 10
&& array1.Contains(u.Id)
&& u.Id + 2 < 10
&& (((object)u.Id).ToString() == "1" || u.Name.Contains("三"))
&& Math.Abs(u.Id) == 1
&& Filter(u.Name)
&& true
;
//=====exp2=====
exp2 = exp.Cast<UserDto, User>();
Console.WriteLine(exp.ToString());
Console.WriteLine(exp.ToString());
//測試數據
List<User> list = new List<User>() {
new User{ Id=0,Name="AAA"},
new User{ Id=1,Name="張三"},
new User{ Id=2,Name="李四"}
};
var item = list.Where(exp2.Compile()).FirstOrDefault();
Console.WriteLine(item.Name);
Console.ReadKey();
}
public static bool Filter(string name)
{
return name.Contains("三");
}
}
應該說常用的篩選條件都是支持的。這裡的list由於沒有數據庫環境就用List<User>模擬的，真實ORM環境換成list.Where(exp2)就可以了。
性能方面沒有測試，應該是可以使用緩存的。有興趣的朋友可以改一下。
