blocks|key|325513|text|您可以解析字符串并使用Expression类构建一个lambda表达式，本质上复制编译器的功能。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|325514|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/system.linq.expressions.expression_members.aspx^0|B|A|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

You could parse the string and build up a lambda expression using the <a href="http://msdn.microsoft.com/en-us/library/system.linq.expressions.expression_members.aspx" rel="noreferrer">Expression</a> class, essentially duplicating the function of the compiler.

blocks|key|325519|text|您可以使用CSharpCodeProvider做一些事情(用一些更多的代码包装表达式，以创建一个有效的类并将其编译成一个程序集，然后加载该程序集)。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|325520|我相信LINQPad就是这么做的。|325521|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/microsoft.csharp.csharpcodeprovider.aspx|1|http://www.linqpad.net/^0|5|I|0|0|3|7|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

You might be able to do something with <a href="http://msdn.microsoft.com/en-us/library/microsoft.csharp.csharpcodeprovider.aspx" rel="nofollow noreferrer">CSharpCodeProvider</a> (wrap the expression with some more code to create a valid class and compile it into an assembly, then load the assembly). 

I believe that is how <a href="http://www.linqpad.net/" rel="nofollow noreferrer">LINQPad</a> does it.

blocks|key|325529|text|我想你不得不求助于CSharpCodeProvider。但是，处理所有可能的局部变量引用可能并不容易。您将如何告诉CSharpCodeProvider有关lambda参数的类型？我可能会创建一个模板类，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|325530|class+ExpressionContainer+{
++++public+Expression<Func<Product,+bool>>+TheExpression;
++++public+string+Length;

++++public+ExpressionContainer()+{
++++++++TheExpression+=+<user+expression+text>;
++++}
}|code-block|syntax|javascript|325531|然后这样做：|325532|string+source+=+<Code+from+above>;
Assembly+a;
using+(CSharpCodeProvider+provider+=+new+CSharpCodeProvider(...)+{
++++List<string>+assemblies+=+new+List<string>();
++++foreach+(Assembly+x+in+AppDomain.CurrentDomain.GetAssemblies())+{
++++++++try+{
++++++++++++assemblies.Add(x.Location);
++++++++}
++++++++catch+(NotSupportedException)+{
++++++++++++//+Dynamic+assemblies+will+throw,+and+in+.net+3.5+there+seems+to+be+no+way+of+finding+out+whether+the+assembly+is+dynamic+before+trying.
++++++++}
++++}

++++CompilerResults+r+=+provider.CompileAssemblyFromSource(new+CompilerParameters(assemblies.ToArray())+{+GenerateExecutable+=+false,+GenerateInMemory+=+true+},+source);
++++if+(r.Errors.HasErrors)
++++++++throw+new+Exception("Errors+compiling+expression:+"+%2B+string.Join(Environment.NewLine,+r.Errors.OfType<CompilerError>().Select(e+=>+e.ErrorText).ToArray()));
++++a+=+r.CompiledAssembly;
}
object+o+=+a.CreateInstance("ExpressionContainer");
var+result+=+(+Expression<Func<Product,+bool>>)o.GetType().GetProperty("TheExpression").GetValue(o);|325533|但是，请注意，对于长时间运行的应用程序，您应该在单独的appdomain中创建所有这些内存中的程序集，因为在卸载它们所在的appdomain之前，它们不能被释放。|325534|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I guess you have to resort to the CSharpCodeProvider. However, dealing with all possible local variable references might not be trivial. And how would you tell the CSharpCodeProvider about the type of the lambda parameter? I would probably create a template class looking like this:

<pre><code>class ExpressionContainer {
 public Expression&lt;Func&lt;Product, bool&gt;&gt; TheExpression;
 public string Length;

 public ExpressionContainer() {
 TheExpression = &lt;user expression text&gt;;
 }
}
</code></pre>

Then do something like this:

<pre><code>string source = &lt;Code from above&gt;;
Assembly a;
using (CSharpCodeProvider provider = new CSharpCodeProvider(...) {
 List&lt;string&gt; assemblies = new List&lt;string&gt;();
 foreach (Assembly x in AppDomain.CurrentDomain.GetAssemblies()) {
 try {
 assemblies.Add(x.Location);
 }
 catch (NotSupportedException) {
 // Dynamic assemblies will throw, and in .net 3.5 there seems to be no way of finding out whether the assembly is dynamic before trying.
 }
 }

 CompilerResults r = provider.CompileAssemblyFromSource(new CompilerParameters(assemblies.ToArray()) { GenerateExecutable = false, GenerateInMemory = true }, source);
 if (r.Errors.HasErrors)
 throw new Exception("Errors compiling expression: " + string.Join(Environment.NewLine, r.Errors.OfType&lt;CompilerError&gt;().Select(e =&gt; e.ErrorText).ToArray()));
 a = r.CompiledAssembly;
}
object o = a.CreateInstance("ExpressionContainer");
var result = ( Expression&lt;Func&lt;Product, bool&gt;&gt;)o.GetType().GetProperty("TheExpression").GetValue(o);
</code></pre>

Note, however, that for long-running applications, you should create all these in-memory assemblies in a separate appdomain since they can't be freed until the appdomain they reside in is unloaded.

blocks|key|328637|text|它们是许多可用的lambda表达式解析器。其中一些是Lambda-Parser、Sprache|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|328638|示例代码：|328639|Example1+:字符串拼接和数字计算：|328640|string+code+=+"2.ToString()%2B(4*2)";+//+C#+code+Func<string>+
func+=+ExpressionParser.Compile<Func<string>>(code);+//+compile+code+
string+result+=+func();+//+result+=+"28"|code-block|syntax|javascript|328641|entityMap|0|LINK|mutability|MUTABLE|url|http://code.google.com/p/lambda-parser/downloads/detail?name=Zhucai.LambdaParser.dll&can=2&q=|1|http://github.com/sprache/sprache^0|Q|D|0|14|7|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]|$A|10|B|11|1|12]]|C|$]]|$1|D|3|E|5|6|7|13|8|@]|9|@]|C|$]]|$1|F|3|G|5|6|7|14|8|@]|9|@]|C|$]]|$1|H|3|I|5|J|7|15|8|@]|9|@]|C|$K|L]]|$1|M|3|-4|5|6|7|16|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]|U|$5|P|Q|R|C|$S|V]]]]

They are many lambda expression parsers available. Some of them are <a href="http://code.google.com/p/lambda-parser/downloads/detail?name=Zhucai.LambdaParser.dll&amp;can=2&amp;q=" rel="nofollow noreferrer">Lambda-Parser</a>, <a href="http://github.com/sprache/sprache" rel="nofollow noreferrer">Sprache</a>

Sample Code:

Example1 : string concat and number calculate: 

<pre><code>string code = "2.ToString()+(4*2)"; // C# code Func&lt;string&gt; 
func = ExpressionParser.Compile&lt;Func&lt;string&gt;&gt;(code); // compile code 
string result = func(); // result = "28"
</code></pre>

blocks|key|3079781|text|对于您的更具体的问题(您可能已经知道这一点，但我将尝试提及它)，您可以创建一个字典，将可以是常量(整数或枚举)的值映射到lambda。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3079782|sealed+class+Product+{
+++public+bool+CanDoThis+{+get;+set;+}
+++public+int+AllowedCount+{+get;+set;+}
}

public+enum+SecureFuncType+{
+++Type1,
+++Type2,
+++Type3
}

sealed+class+SecureAttribute+:+Attribute+{
+++[NotNull]+readonly+Func<Product,+bool>+mFunc;

+++public+SecureAttribute(SecureFuncType+pType)+{
++++++var+secureFuncs+=+new+Dictionary<SecureFuncType,+Func<Product,+bool>>+{
+++++++++{+SecureFuncType.Type1,+role+=>+role.CanDoThis+&&+role.AllowedCount+>+1+},
+++++++++{+SecureFuncType.Type2,+role+=>+role.CanDoThis+&&+role.AllowedCount+>+2+},
+++++++++{+SecureFuncType.Type3,+role+=>+role.CanDoThis+&&+role.AllowedCount+>+3+}
++++++};

++++++mFunc+=+secureFuncs[pType];
+++}
}|code-block|syntax|javascript|3079783|​|3079784|[Secure(SecureFuncType.Type1)]
sealed+class+TestClass+{
}

//+etc...|3079785|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Responding to your more specific problem, (and you may already know this but I'll try mentioning it anyway), you could create a dictionary that maps values that can be a constant (integers or enums) to lambdas.

<pre><code>sealed class Product {
 public bool CanDoThis { get; set; }
 public int AllowedCount { get; set; }
}

public enum SecureFuncType {
 Type1,
 Type2,
 Type3
}

sealed class SecureAttribute : Attribute {
 [NotNull] readonly Func&lt;Product, bool&gt; mFunc;

 public SecureAttribute(SecureFuncType pType) {
 var secureFuncs = new Dictionary&lt;SecureFuncType, Func&lt;Product, bool&gt;&gt; {
 { SecureFuncType.Type1, role =&gt; role.CanDoThis &amp;&amp; role.AllowedCount &gt; 1 },
 { SecureFuncType.Type2, role =&gt; role.CanDoThis &amp;&amp; role.AllowedCount &gt; 2 },
 { SecureFuncType.Type3, role =&gt; role.CanDoThis &amp;&amp; role.AllowedCount &gt; 3 }
 };

 mFunc = secureFuncs[pType];
 }
}
</code></pre>



<pre><code>[Secure(SecureFuncType.Type1)]
sealed class TestClass {
}

// etc...
</code></pre>

Is there a way to convert string representation of lambda to a lambda Func?

<pre><code>Func&lt;Product, bool&gt; func = Parse&lt;Product, bool&gt;("product =&gt; product.Name.Length &gt; 0");
</code></pre>

I tried Dynamic LINQ but it doesn't work as expected - for example it doesn't expect lambda syntax =>.

Summary of answers:

<ul>
<li>writing my own C# compiler - very funny</li>
<li>firing up external compiler (like csc.exe) - very slow</li>
<li>using DLINQ - as I said I don't see how it can parse lambda expressions</li>
</ul>

Why do I need this: because there's no way to pass lambdas to custom attributes like

<pre><code>[Secure(role =&gt; role.CanDoThis &amp;&amp; role.AllowedCount &gt; 5)]
</code></pre>

So as a workaround I'd like to pass lambda as string: "role => role.CanDoThis &amp;&amp; role.AllowedCount > 5". But seems like I'll have to use DLINQ like this: "CanDoThis &amp;&amp; AllowedCount > 5" - since that's the syntax it understands. But my question was about true lambdas, I've already used DLINQ at the time of asking.

Parse string to C# lambda Func

编译器

有没有办法将lambda的字符串表示转换为lambda Func？Func<Product, bool> func = Parse<Product, bool>("product => product.Name.Length > 0");我尝试过Dynamic LINQ，但是它不能像预期的那样工作--例如，它不需要la...

问将字符串解析为C#λ函数
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