我该如何使用表达式树动态创建类似以下谓词的断言?...
(p.Length== 5) && (p.SomeOtherProperty == "hello")
以便我可以将谓词转换为类似于lambda表达式的形式...
q.Where(myDynamicExpression)...
我只需要指出正确的方向。
更新: 抱歉,各位,我漏掉了一个事实,那就是我希望谓词具有多个条件,如上所述。对于造成的混乱,我感到抱歉。
我该如何使用表达式树动态创建类似以下谓词的断言?...
(p.Length== 5) && (p.SomeOtherProperty == "hello")
以便我可以将谓词转换为类似于lambda表达式的形式...
q.Where(myDynamicExpression)...
我只需要指出正确的方向。
更新: 抱歉,各位,我漏掉了一个事实,那就是我希望谓词具有多个条件,如上所述。对于造成的混乱,我感到抱歉。
原始内容
像这样:
var param = Expression.Parameter(typeof(string), "p");
var len = Expression.PropertyOrField(param, "Length");
var body = Expression.Equal(
len, Expression.Constant(5));
var lambda = Expression.Lambda<Func<string, bool>>(
body, param);
更新
关于 (p.Length== 5) && (p.SomeOtherProperty == "hello")
:
var param = Expression.Parameter(typeof(SomeType), "p");
var body = Expression.AndAlso(
Expression.Equal(
Expression.PropertyOrField(param, "Length"),
Expression.Constant(5)
),
Expression.Equal(
Expression.PropertyOrField(param, "SomeOtherProperty"),
Expression.Constant("hello")
));
var lambda = Expression.Lambda<Func<SomeType, bool>>(body, param);
&&
运算符将多个谓词组合在一起时,需要每次两个两个地连接它们。predicates
的Expression对象列表,请执行以下操作:Expression combined = predicates.Aggregate((l, r) => Expression.AndAlso(l, r));
// Usage first.Compose(second, Expression.And)
public static Expression<T> Compose<T>(this Expression<T> First, Expression<T> Second, Func<Expression, Expression, Expression> Merge)
{
// build parameter map (from parameters of second to parameters of first)
Dictionary<ParameterExpression,ParameterExpression> map = First.Parameters.Select((f, i) => new { f, s = Second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
// replace parameters in the second lambda expression with parameters from the first
Expression secondBody = ParameterRebinder.ReplaceParameters(map, Second.Body);
// apply composition of lambda expression bodies to parameters from the first expression
return Expression.Lambda<T>(Merge(First.Body, secondBody), First.Parameters);
}
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> First, Expression<Func<T, bool>> Second)
{
return First.Compose(Second, Expression.And);
}
public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> First, Expression<Func<T, bool>> second)
{
return First.Compose(second, Expression.Or);
}
public class ParameterRebinder : ExpressionVisitor
{
private readonly Dictionary<ParameterExpression, ParameterExpression> map;
public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
{
this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
}
public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
{
return new ParameterRebinder(map).Visit(exp);
}
protected override Expression VisitParameter(ParameterExpression p)
{
ParameterExpression replacement;
if (map.TryGetValue(p, out replacement))
{
p = replacement;
}
return base.VisitParameter(p);
}
}
我有一个名为Exprelsior的开源项目,它提供了非常简单的方法来创建动态谓词:
根据你的例子:
var exp1 = ExpressionBuilder.CreateBinary<YourClass>("MyProperty.Length", 5, ExpressionOperator.Equals);
var exp2 = ExpressionBuilder.CreateBinary<YourClass>("SomeOtherProperty", "hello", ExpressionOperator.Equals);
var fullExp = exp1.And(exp2);
// Use it normally...
q.Where(fullExp)
var exp1 = ExpressionBuilder.CreateBinaryFromQuery<YourClass>("eq('MyProperty.Length', '5')");
var exp2 = ExpressionBuilder.CreateBinaryFromQuery<YourClass>("eq('SomeOtherProperty', 'hello')");
var fullExp = exp1.And(exp2);
// Use it normally...
q.Where(fullExp)
它支持更多的数据类型和运算符。
SomeType
,我们怎么能创建lambda表达式呢?例如:我们只有类型TyepOfEntity = Assembly.GetType(string.Format("Smartiz.Data.{0}", EntityName));
。 - Mohammad Dayyan