如何从Expression <Func <MyClass,string >>动态创建Expression <Func <MyClass,bool >>谓词？

Question

我试图追加谓词和我的目标是创建相同的表达式:

Services.Where(s => s.Name == "Modules" && s.Namespace == "Namespace");

我有以下代码:

Expression<Func<Service,string>> sel1 = s => s.Name;
Expression<Func<Service,string>> sel2 = s => s.Namespace;

var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");

Expression e1 = Expression.Equal(sel1.Body, val1);
Expression e2 = Expression.Equal(sel2.Body, val2);
var andExp = Expression.AndAlso(e1, e2);

ParameterExpression argParam = Expression.Parameter(typeof(string), "s");
var lambda = Expression.Lambda<Func<string, bool>>(andExp, argParam);

这将创建以下输出:

s => ((s.Name == "Modules") AndAlso (s.Namespace == "Namespace"))

但是,这是错误的,因为Name和Namespace的参数不相同.如果我将其中一个表达式选择器更改为:

Expression<Func<Service,string>> sel2 = srv => srv.Namespace;

输出将是:

s => ((s.Name == "Modules") AndAlso (srv.Namespace == "Namespace"))

如何使用sel1和sel2创建有效的表达式？

更新(2011年2月28日)

我通过创建调用表达式来解决它:Expression.Invoke所以lambda表达式sel1和sel2不一定需要是MemberExpression:

Expression<Func<Service,string>> sel1 = s => s.Name;
Expression<Func<Service,string>> sel2 = srv => srv.Namespace;

var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");

Expression<Func<Service, bool>> lambda = m => true;
var modelParameter = lambda.Parameters.First();

// sel1 predicate
{
    var invokedExpr = Expression.Invoke(sel1, modelParameter);
    var binaryExpression = Expression.Equal(invokedExpr, val1);
    lambda = Expression.Lambda<Func<Service, bool>>(Expression.AndAlso(binaryExpression, lambda.Body), lambda.Parameters);
}
// sel2 predicate
{
    var invokedExpr = Expression.Invoke(sel2, modelParameter);
    var binaryExpression = Expression.Equal(invokedExpr, val2);
    lambda = Expression.Lambda<Func<Service, bool>>(Expression.AndAlso(binaryExpression, lambda.Body), lambda.Parameters);
}

Answer 1

很难混合编译器生成的表达式树和手工制作的表达式树,正是因为这种事情 - 提取出ParameterExpressions是棘手的.所以让我们从头开始:

ParameterExpression argParam = Expression.Parameter(typeof(Service), "s");
Expression nameProperty = Expression.Property(argParam, "Name");
Expression namespaceProperty = Expression.Property(argParam, "Namespace");

var val1 = Expression.Constant("Modules");
var val2 = Expression.Constant("Namespace");

Expression e1 = Expression.Equal(nameProperty, val1);
Expression e2 = Expression.Equal(namespaceProperty, val2);
var andExp = Expression.AndAlso(e1, e2);

var lambda = Expression.Lambda<Func<Service, bool>>(andExp, argParam);

我改变的一个重要方面是传递给Expression.Parameter它的类型- 它看起来应该是一个Service而不是一个string.

我已经尝试了一下,当我lambda.Compile在一些样本Service对象上调用并执行它时,它似乎有效...

Answer 2

您可以为可为空类型创建表达式树，假设您有一个可为空字段 BoardId，您可以像这样动态创建表达式树

var nameValue="BoardId=111";

您需要确定第一个属性类型，无论其是否可为空

下面的代码为可空和不可空类型创建动态树表达式

 public static Expression<Func<T, bool>> BuildWhereExpression<T>(string nameValueQuery ) where  T : class 
        {
            Expression<Func<T, bool>> predicate = null;
            PropertyInfo prop = null;
            var fieldName = nameValueQuery.Split("=")[0];
            var fieldValue = nameValueQuery.Split("=")[1];
            var properties = typeof(T).GetProperties();
            foreach (var property in properties)
            {
                if (property.Name.ToLower() == fieldName.ToLower())
                {
                    prop = property;
                }
            } 
            if (prop != null)
            {
                var isNullable = prop.PropertyType.IsNullableType();
                var parameter = Expression.Parameter(typeof(T), "x");
                var member = Expression.Property(parameter, fieldName); 

                if (isNullable)
                {
                    var filter1 =
                        Expression.Constant(
                            Convert.ChangeType(fieldValue, member.Type.GetGenericArguments()[0]));
                    Expression typeFilter = Expression.Convert(filter1, member.Type);
                    var body = Expression.Equal(member, typeFilter);  
                    predicate = Expression.Lambda<Func<T, bool>>(body, parameter);  
                }
                else
                {
                    if (prop.PropertyType == typeof(string) && likeOerator.ToLower() == "like")
                    {
                        var parameterExp = Expression.Parameter(typeof(T), "type");
                        var propertyExp = Expression.Property(parameterExp, prop);
                        MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
                        var someValue = Expression.Constant(fieldValue, typeof(string));
                        var containsMethodExp = Expression.Call(propertyExp, method, someValue);
                        predicate = Expression.Lambda<Func<T, bool>>(containsMethodExp, parameterExp);
                    }
                    else
                    {
                        var constant = Expression.Constant(Convert.ChangeType(fieldValue, prop.PropertyType));
                        var body = Expression.Equal(member, constant);  
                        predicate = Expression.Lambda<Func<T, bool>>(body, parameter); `enter code here`
                    }
                }
            }
            return predicate;
        }

1-此解决方案首先检查 Nullable 值并生成表达式。这是确定类型是否可为 Null 的方法。我为此目的创建了一个扩展方法

  public static bool IsNullableType(this Type type) {  return
    type.IsGenericType &&
    (type.GetGenericTypeDefinition().Equals(typeof(Nullable<>))); }

2-第二步是检查其字符串的类型，然后为字符串创建一个表达式。

3-第三步是检查值是否不可为空，不是字符串，然后使用 equal 创建一个表达式