使用lambda表达式来避免使用“魔术字符串”以指定属性 [英] Using a lambda expression to avoid using a "magic string" to specify a property
问题描述
我正在写一个服务来收集一个特定类型的对象集合将其原始,字符串和DateTime类型输出到 CSV格式。我有两个以下的语句工作。
$ b pre>
string csv = new ToCsvService< DateTime>(objs)
.Exclude(Minute)
.ChangeName(Millisecond,Milli)
.Format(Date,d)
.ToCsv();
vs。 Lambda版本
string csv = new ToCsvService< DateTime>(objs)
.Exclude(p = > p.Minute)
.ChangeName(p => p.Millisecond,Milli)
.Format(p => p.Date,d)
.ToCsv ();
根据Jon Skeet的建议,所有lambda方法共享类似的方法签名
public IToCsvService< T>排除< TResult>(
表达式< Func< T,TResult>>表达式)
然后我将 expression.Body
传递给 FindMemberExpression
。我已修改 <$ c nhlambdaextensions中的$ c> FindMemberExpression 的ExpressionProcessor.cs 方法项目。我的非常相似的版本的 FindMemberExpression
如下:
private string FindMemberExpression表达式)
{
if(表达式为MemberExpression)
{
MemberExpression memberExpression =(MemberExpression)expression;
if(memberExpression.Expression.NodeType == ExpressionType.MemberAccess
|| memberExpression.Expression.NodeType == ExpressionType.Call)
{
if(memberExpression。 Member.DeclaringType.IsGenericType
&& memberExpression.Member.DeclaringType
.GetGenericTypeDefinition()。等于(typeof(Nullable<>)))
{
if Value.Equals(memberExpression.Member.Name))
{
return FindMemberExpression(memberExpression.Expression);
}
return String.Format({0}。{1},
FindMemberExpression(memberExpression.Expression),
memberExpression.Member.Name);
}
}
else
{
return memberExpression.Member.Name;
}
}
throw new Exception(Could not determine member from
+ expression.ToString());
}
我在 FindMemberExpression
?给定我的用例,我在做什么overkill?
编辑:使这更简单的核心是更改签名你的方法在结果类型中是通用的:
public IToCsvService< TSource>排除< TResult>(
Expression< Func< TSource,TResult>>表达式)
<这样,你不会得到一个转换表达式,因为不需要转换。例如, p => p.Minute
将由于类型推断而自动以表达式< Func< DateTime,int>>
$ b
这看起来像对我太过分了,因为现在所有你需要的是一个财产 - 至少,这是你的样品显示。 / p>
为什么不开始只是识别某个属性,如果需要可以稍后再展开?
编辑:这是一个简短但完整的示例,不显示任何转换:
using System;
使用System.Linq.Expressions;
class Test
{
static void Main()
{
Expression< Func< DateTime,int> dt = p =分钟;
Console.WriteLine(dt);
}
}
如果将表达式类型更改为 Expression
但是,显示 Convert(...)
位。我怀疑您需要更改排除
(etc)方法的签名。
I am writing a service to take a collection of objects of a particular type and output its primitive, string, and DateTime types to a string in CSV Format. I have both of the below statements working. I find the the lambda based version to be much cleaner.
Magic String Version
string csv = new ToCsvService<DateTime>(objs)
.Exclude("Minute")
.ChangeName("Millisecond", "Milli")
.Format("Date", "d")
.ToCsv();
vs. Lambda Version
string csv = new ToCsvService<DateTime>(objs)
.Exclude(p => p.Minute)
.ChangeName(p => p.Millisecond, "Milli")
.Format(p => p.Date, "d")
.ToCsv();
Per Jon Skeet's recommendation all of the lambda methods share a similar method signature
public IToCsvService<T> Exclude<TResult>(
Expression<Func<T, TResult>> expression)
I then pass the expression.Body
to FindMemberExpression
. I've adapted code from the FindMemberExpression
method of ExpressionProcessor.cs from the nhlambdaextensions project. My very similar version of FindMemberExpression
is below:
private string FindMemberExpression(Expression expression)
{
if (expression is MemberExpression)
{
MemberExpression memberExpression = (MemberExpression)expression;
if (memberExpression.Expression.NodeType == ExpressionType.MemberAccess
|| memberExpression.Expression.NodeType == ExpressionType.Call)
{
if (memberExpression.Member.DeclaringType.IsGenericType
&& memberExpression.Member.DeclaringType
.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
{
if ("Value".Equals(memberExpression.Member.Name))
{
return FindMemberExpression(memberExpression.Expression);
}
return String.Format("{0}.{1}",
FindMemberExpression(memberExpression.Expression),
memberExpression.Member.Name);
}
}
else
{
return memberExpression.Member.Name;
}
}
throw new Exception("Could not determine member from "
+ expression.ToString());
}
I am testing for enough cases in FindMemberExpression
? Is what I am doing overkill given my use case?
EDIT: The core to making this simpler is to change the signature of your methods to be generic in the result type too:
public IToCsvService<TSource> Exclude<TResult>(
Expression<Func<TSource, TResult>> expression)
That way you won't end up with a conversion expression because no conversion will be necessary. For example, p => p.Minute
will end up as an Expression<Func<DateTime, int>>
automatically due to type inference.
It looks like overkill to me, given that at the moment all you need is a property - at least, that's all that your sample shows.
Why not start off just recognising a property, and expand it later if you need to?
EDIT: Here's a short but complete example which doesn't show any conversions:
using System;
using System.Linq.Expressions;
class Test
{
static void Main()
{
Expression<Func<DateTime, int>> dt = p => p.Minute;
Console.WriteLine(dt);
}
}
If you change the expression type to Expression<Func<DateTime, long>>
however, it does show the Convert(...)
bit. I suspect you need to change the signatures of your Exclude
(etc) methods.
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