T>我怎么能对一个IEnumerable<创建一个动态的选择;在运行时? [英] How can I create a dynamic Select on an IEnumerable<T> at runtime?
问题描述
在的IEnumerable< T>
,其中 T
的任何对象,我怎么能选择一个?从它特定的属性,因为我知道在运行时作为字符串的属性名称之一的名称
例如:
VAR externalIEnumerable = DataPassedFromConsumingCode(); //`的IEnumerable< T>`
串knownPropertyName =富;
VAR fooSelect = externalIEnumerable.Select(...);
在本质上,我明明只是做 externalIEnumerable.Select(X = > x.Foo);
,但我需要执行此选择
在运行时,当我没有当它最初创建了控制
-
答:基于AlanT的回答,这就是我实际上所做的:
公共表达式来; Func键< TItem,对象>> SelectExpression< TItem>(串字段名)
{
VAR参数= Expression.Parameter(typeof运算(TItem),项目);
VAR字段= Expression.Property(参数,字段名);
返回Expression.Lambda<&Func键LT; TItem,对象>>(场,
新ParameterExpression [] {参数});
}
我一直是作为一个表达式,因为调用编译
引起的IQueryable被枚举,这意味着该数据库不必要的打击。所以,使用它,我只是做了以下内容:
字符串的PrimaryKey = _map.GetPrimaryKeys(typeof运算(TOriginator))单();
VAR primaryKeyExpression = SelectExpression< TOriginator>(PrimaryKey的);
VAR primaryKeyResults = query.Select(primaryKeyExpression).ToList();
这是可能做到这一点使用表达式
如:
私有类Foo {
酒店的公共串酒吧{搞定;组; }
}
私人的IEnumerable<富> SomeFoos =新的List<富>(){
新富{吧=一月},
新富{吧=二月},
新富{吧=三月 }
新富{吧=四月},
};
[TestMethod的]
公共无效GetDynamicProperty(){
VAR EXPR = SelectExpression<富,串>(酒吧);
VAR propValues = SomeFoos.Select(表达式);
Assert.IsTrue(新[] {一月,二月,月,月} .SequenceEqual(propValues));
}
公共静态Func键< TItem,与TFIELD GT; SelectExpression< TItem,与TFIELD GT;(串字段名){
VAR参数= Expression.Parameter(typeof运算(TItem),项目);
VAR字段= Expression.Property(参数,字段名);
返回Expression.Lambda&所述; Func键&下; TItem,TFIELD>>(场,新ParameterExpression [] {参数})编译();
}
心连心,
艾伦。
Given that I have an IEnumerable<T>
, where T
is any object, how can I select a specific property from it, given that I know the name of the one of the property names at run time as a string?
For example:
var externalIEnumerable = DataPassedFromConsumingCode(); // `IEnumerable<T>`
string knownPropertyName = "Foo";
var fooSelect = externalIEnumerable.Select(...);
In essence, I'm obviously just doing externalIEnumerable.Select(x=> x.Foo);
, but I need to perform this Select
at runtime, when I don't have control over when it's initially created.
--
ANSWER: Based on AlanT's answer, here's what I actually did:
public Expression<Func<TItem, object>> SelectExpression<TItem>(string fieldName)
{
var param = Expression.Parameter(typeof(TItem), "item");
var field = Expression.Property(param, fieldName);
return Expression.Lambda<Func<TItem, object>>(field,
new ParameterExpression[] { param });
}
I kept it as an Expression, because calling Compile
caused the IQueryable to be Enumerated, which meant the database was hit unnecessarily. So, to use it, I just do the following:
string primaryKey = _map.GetPrimaryKeys(typeof(TOriginator)).Single();
var primaryKeyExpression = SelectExpression<TOriginator>(primaryKey);
var primaryKeyResults = query.Select(primaryKeyExpression).ToList();
It is possible to do this using an Expression
e.g.
private class Foo {
public string Bar { get; set; }
}
private IEnumerable<Foo> SomeFoos = new List<Foo>() {
new Foo{Bar = "Jan"},
new Foo{Bar = "Feb"},
new Foo{Bar = "Mar"},
new Foo{Bar = "Apr"},
};
[TestMethod]
public void GetDynamicProperty() {
var expr = SelectExpression<Foo, string>("Bar");
var propValues = SomeFoos.Select(expr);
Assert.IsTrue(new[] { "Jan", "Feb", "Mar", "Apr" }.SequenceEqual(propValues));
}
public static Func<TItem, TField> SelectExpression<TItem, TField>(string fieldName) {
var param = Expression.Parameter(typeof(TItem), "item");
var field = Expression.Property(param, fieldName);
return Expression.Lambda<Func<TItem, TField>>(field, new ParameterExpression[] { param }).Compile();
}
hth,
Alan.
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