I have a requirement to modify a method so that it has an extra parameter that will take a lambda expression that will be used on an internal object to return the value of the given property. Forgive my probable incorrect use of terminology as this is my first foray into LINQ expressions!
I have tried searching for an answer, but as I mentioned, my terminology seems to be off and the examples I can find are far too complex or deal with expressions for collection functions such as .Where(), which I am familiar with.
What I have so far (cut down version):
class MyClass
{
private MyObject _myObject = new MyObject() { Name = "Test", Code = "T" };
private string MyMethod(int testParameter, ??? selector)
{
//return _myObject.Name;
//return _myObject.Code;
return ???;
}
}
I would like to call it something like this:
string result = _myClassInstance.MyMethod(1, (x => x.Name));
or:
string result = _myClassInstance.MyMethod(1, (x => x.Code));
Obviously the parts which I am missing is the selector parameter in MyMethod, how to apply it to the local variable and how to pass the required property into the method when I am invoking it.
Any help would be appreciated, also extra bonus points for a VB.NET solutions as well as unfortunately the final implementation needs to be in our lone VB project!
private string MyMethod(int testParameter, Func<MyObject, string> selector)
{
return selector(_myObject);
}
When using Func delegates, the last parameter is the return type and the first N-1 are the argument types. In this case, there is a single MyObject argument to selector and it returns a string.
You can invoke it like:
string name = _myClassInstance.MyMethod(1, x => x.Name);
string result = _myClassInstance.MyMethod(1, x => x.Code);
Since the return type of MyMethod matches the return type of your selector delegate, you could make it generic:
private T MyMethod<T>(int testParameter, Func<MyObject, T> selector)
{
MyObject obj = //
return selector(obj);
}
I don't know VB.Net but it looks like it would be:
Public Function MyMethod(testParameter as Integer, selector as Func(Of MyObject, String))
Return selector(_myObject)
End Function
and the generic version would be:
Public Function MyMethod(Of T)(testParameter as Integer, selector Func(Of MyObject, T))
Return selector(_myObject)
End Function
I will show you a different approach that is very flexible (see DotNetFiddle at the bottom): You can easily write your own LINQ functions to extend existing functions or write your own functions and benefit from the power of LINQ queries.
In this example, I am improving Linq's Distinct function in a way so you can specify a field, which is used for grouping.
Usage (Example):
var myQuery=(from x in Customers select x).MyDistinct(d => d.CustomerID);
In this example the query is being grouped by CustomerID and the first element of each group is returned.
Declaration of MyDistinct:
public static class Extensions
{
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
Func<T, V> f)
{
return query.GroupBy(f).Select(x=>x.First());
}
}
You can see that f, the 2nd parameter, is declared as Func<T, V>, so it can be used by the .GroupBy statement.
Coming back to the code in your question, if you have declared
class MyObject
{
public string Name;
public string Code;
}
private MyObject[] _myObject = {
new MyObject() { Name = "Test1", Code = "T"},
new MyObject() { Name = "Test2", Code = "Q"},
new MyObject() { Name = "Test2", Code = "T"},
new MyObject() { Name = "Test5", Code = "Q"}
};
you could use that with the newly defined function MyDistinct as follows:
var myQuery = (from x in _myObject select x).MyDistinct(d => d.Code);
which will return
Name Code
Test1 T
Test2 Q
or you can use .MyDistinct(d => d.Name) in the query, which returns:
Name Code
Test1 T
Test2 Q
Test5 Q
Notice that because MyDistinct is declared with the generics T and V, it recognizes and uses the right object types automatically and returns MyObject elements.
Advanced usage
Notice that MyDistinct always takes the first element of each group. What if you need a condition defining which element you need?
Here's how you can do it:
public static class Extensions
{
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
Func<T, V> f,
Func<IGrouping<V,T>,T> h=null)
{
if (h==null) h=(x => x.First());
return query.GroupBy(f).Select(h);
}
}
This modification either allows you to use it exactly as before, i.e. by specifying one parameter like .MyDistinct(d => d.Name), but it also allows you to specify a having condition such as x => x.FirstOrDefault(y => y.Name.Contains("1")||y.Name.Contains("2")) as a second parameter like so:
var myQuery2 = (from x in _myObject select x).MyDistinct(d => d.Name,
x=>x.FirstOrDefault(y=>y.Name.Contains("1")||y.Name.Contains("2"))
);
If you run this query, the result is:
Name Code
Test1 T
Test2 Q
null
because Test5 does not meet the condition (it does not contain 1 or 2), you're getting null in the 3rd row.
Note: If you want to expose just the condition, you can have it even simpler by implementing it as:
public static IEnumerable<T> MyDistinct2<T, V>(this IEnumerable<T> query,
Func<T, V> f,
Func<T,bool> h=null
)
{
if (h == null) h = (y => true);
return query.GroupBy(f).Select(x=>x.FirstOrDefault(h));
}
In this case, the query would just look like:
var myQuery3 = (from x in _myObject select x).MyDistinct2(d => d.Name,
y => y.Name.Contains("1") || y.Name.Contains("2")
);
so you don't need to write x=>x.FirstOrDefault(... condition ...).
Try it in DotNetFiddle
in C#
The parameter type you are looking for Func
private string MyMethod(int testParameter, Func<MyClass,string> selector){
return selector(_myObject);
}
in VB you still want Func the syntax is a little different.
Function MyMethod(ByVal testParameter As Integer, ByVal selector as Func(Of MyClass,string) as string
return selector(_myObject)
End Function
class MyClass
{
private MyObject _myObject = new MyObject() { Name = "Test", Code = "T" };
private string MyMethod(int testParameter, Func<MyObject, string> selector)
{
return selector(_myObject );
}
}
You can do that with a delegate of your selector:
delegate string SampleDelegate(MyObject obj);
private string MyMethod(int testParameter, SampleDelegate selector)
{
return selector(_myObject);
}
You are probably looking for the Delegate class ("Delegate" in VB, "delegate" in C#), or one of its subtypes.
This page has some examples you will probably find useful, especially near the bottom of the page.
Here is a VB example of what you would want to do:
Public Class MyClass
Private Property _myObject As MyObject = New MyObject With {.Name = "Test", .Code = "T"}
Private Function MyMethod(testParameter As Integer, selector As Func(Of MyObject, String)) As String
Return selector(_myObject).ToString
End Function
End Class
Related
I have read about Func<>, which says it iss a delegate and you can use it like for example:
Func<class,bool>
means you send a class or anything and get a bool result, this is what I got!
but what does the following mean?
Func<Class, bool> predicate
I have no idea, can you make it clear for me?
The former will not compile since class is a registered keyword and can only be used for class definitions.
The latter is a Func<T, TResult> which is a function that takes a parameter of type T and returns an object of type TResult. So in your case, the function takes an object of type Class and returns a bool.
The naming of Class is unfortunate but it’s actually allowed to use that identifier for a type. The following would be an example that takes such an object of type Class and then checks a property of it, returning true or false depending on the success of the comparison.
Func<Class,bool> predicate = obj => obj.Prop == "Foo";
var x = new Class();
x.Prop = "Foo";
Console.WriteLine(predicate(x)); // true
// with the following class definition
public class Class
{
public string Prop { get; set; }
}
A Func<> object is callable, just like methods, so we can use parentheses to call it and pass the object here. This would be equivalent to a method like this:
public bool predicate(Class obj)
{
return obj.Prop == "Foo";
}
But the usual way to use Func<> is using lambda expressions to be able to create functions quickly inline. This is very commonly used in LINQ where you use lambda expressions, and as such Func<> objects, all the time:
var filtered = listOfOjects.Where(obj => obj.Prop == "Foo").ToList();
// ^^^^^^^^^^^^^^^^^^^^^^^^
// this is the predicate function from above
// so you could also pass the predicate Func from above:
var filtered = listOfObjects.Where(predicate).ToList()
Func<MyClass,bool> is a delegate type
In Func<MyClass, bool> predicate , predicate is a delegate variable.
You would normally see something like
Func<MyClass, bool> predicate = c => c.IsValid;
...
if (predicate(myClass1)) DoSomething();
That's just how you instantiate the Func. Compare to a string:
string str;
If you want to instantiate and assign it at the same time, you do something like this:
Func<string, bool> isLongerThanThree = input => input.Length > 3;
isLongerThanThree("string"); // Returns "true"
Are you referring to the word predicate?
That's just the name of a parameter. Notice the similarity:
MyMethod(int myAge)
MyMethod(List<bool> myBooleans)
MyMethod(Func<Class,bool> myPredicate)
Also notice the similarities between:
int myAge = 30;
myAge is a variable of type int who has been given the value of 30.
Func<Class,bool> myPredicate = (x => x.IsAlive);
myPredicate is a variable of type Func<Class,bool> who has been given the value of (x => x.IsAlive).
Func<> is a delegate which represents a method which return a result. C# provides signatures for up to 15 input arguments, which should be enough to represent all possible methods you will ever need :)
it is hardly event to imagine method which has 15 input arguments. Beside Func<> there are also some special version of the delegate like Predicate<T> which is nothing else that Func<in T, bool TResult> or Action<> which represent a function without return value, return value is void.
Delegates(C# Programming Guide)
You can assign any method, static or instance or even anonymous with the matching signature to the Func<>. For example:
Func<MyClass, bool> predicate = (myClass) => {return true;} //anonymoys method
public class MyClass
{
public bool MyPredicate(MyClass myClass)
{
return true;
}
public static bool MyStaticPredicate(MyClass myClass)
{
return true;
}
}
Func<MyClass, bool> predicate = new MyClass().MyPredicate;
Func<MyClass, bool> staticPredicate = MyClass.MyStaticPredicate;
Suppose I have a complex lambda expression as follows:
x => x.A.HasValue || (x.B.HasValue && x.C == q) || (!x.C.HasValue && !x.A.HasValue) || //...expression goes on
I want to use this as an Expression<Func<T,bool> in (e.g. Linq-To-Entities) Queryable.Where method. I also want to use it in the Enumerable.Where method, but the Where method only accepts a Func<T,bool>, not an Expression<Func<T,bool>.
The lambda syntax itself can be used to generate either an Expression<Func<T,bool>> or a Func<T,bool> (or any delegate type for that matter), but in this context it cannot generate more than one at once.
For example, I can write:
public Expression<Func<Pair,bool>> PairMatchesExpression()
{
return x => x.A == x.B;
}
as easily as I can write:
public Func<Pair,bool> PairMatchesDelegate()
{
return x => x.A == x.B;
}
The problem is that I cannot use the same exact lambda expression (i.e. x => x.A == x.B) in both ways, without physically duplicating it into two separate methods with two different return types, in spite of the compiler's ability to compile it into either one.
In other words, if I'd like to use the lambda expression in the Queryable methods, then I have to use the Expression method signature. Once I do that however, I cannot use it as a Func as easily as I could have had I just declared the method return type as Func. Instead, I now have to call Compile on the Expression and then worry about caching the results manually like so:
static Func<Pair,bool> _cachedFunc;
public Func<Pair,bool> PairMatchesFunc()
{
if (_cachedFunc == null)
_cachedFunc = PairMatchesExpression().Compile();
return _cachedFunc;
}
Is there a solution to this problem so that I can use the lambda expression in a more general way without it being locked down to a particular type at compile-time?
Unfortunately, I can see no way to truly get, at compile time, a Func and an Expression from the same lambda. However, you could at least encapsulate away the difference, and you can also defer the compilation of the Func until the first time it's used. Here's a solution that makes the best of things and may meet your needs, even though it doesn't quite go all the way to what you really wanted (compile-time evaluation of both the Expression and the Func).
Please note that this works fine without using the [DelegateConstraint] attribute (from Fody.ExtraConstraints), but with it, you will get compile-time checking of the constructor parameter. The attributes make the classes act like they have a constraint where T : Delegate, which is not currently supported in C#, even though it is supported in the ILE (not sure if I'm saying that right, but you get the idea).
public class VersatileLambda<[DelegateConstraint] T> where T : class {
private readonly Expression<T> _expression;
private readonly Lazy<T> _funcLazy;
public VersatileLambda(Expression<T> expression) {
if (expression == null) {
throw new ArgumentNullException(nameof(expression));
}
_expression = expression;
_funcLazy = new Lazy<T>(expression.Compile);
}
public static implicit operator Expression<T>(VersatileLambda<T> lambda) {
return lambda?._expression;
}
public static implicit operator T(VersatileLambda<T> lambda) {
return lambda?._funcLazy.Value;
}
public Expression<T> AsExpression() { return this; }
public T AsLambda() { return this; }
}
public class WhereConstraint<[DelegateConstraint] T> : VersatileLambda<Func<T, bool>> {
public WhereConstraint(Expression<Func<T, bool>> lambda)
: base(lambda) { }
}
The beauty of the implicit conversion is that in contexts where a specific Expression<Func<>> or Func<> is expected, you don't have to do anything at all, just, use it.
Now, given an object:
public partial class MyObject {
public int Value { get; set; }
}
That is represented in the database like so:
CREATE TABLE dbo.MyObjects (
Value int NOT NULL CONSTRAINT PK_MyObjects PRIMARY KEY CLUSTERED
);
Then it works like this:
var greaterThan5 = new WhereConstraint<MyObject>(o => o.Value > 5);
// Linq to Objects
List<MyObject> list = GetObjectsList();
var filteredList = list.Where(greaterThan5).ToList(); // no special handling
// Linq to Entities
IQueryable<MyObject> myObjects = new MyObjectsContext().MyObjects;
var filteredList2 = myObjects.Where(greaterThan5).ToList(); // no special handling
If implicit conversion isn't suitable, you can cast explicitly to the target type:
var expression = (Expression<Func<MyObject, bool>>) greaterThan5;
Note that you don't really need the WhereConstraint class, or you could get rid of VersatileLambda by moving its contents to WhereConstraint, but I liked making the two separate (as now you can use VersatileLambda for something that returns other than a bool). (And this difference is largely what sets apart my answer from Diego's.) Using VersatileLambda as it is now looks like this (you can see why I wrapped it):
var vl = new VersatileLambda<Func<MyObject, bool>>(o => o.Value > 5);
I have confirmed that this works perfectly for IEnumerable as well as IQueryable, properly projecting the lambda expression into the SQL, as proven by running SQL Profiler.
Also, you can do some really cool things with expressions that can't be done with lambdas. Check this out:
public static class ExpressionHelper {
public static Expression<Func<TFrom, TTo>> Chain<TFrom, TMiddle, TTo>(
this Expression<Func<TFrom, TMiddle>> first,
Expression<Func<TMiddle, TTo>> second
) {
return Expression.Lambda<Func<TFrom, TTo>>(
new SwapVisitor(second.Parameters[0], first.Body).Visit(second.Body),
first.Parameters
);
}
// this method thanks to Marc Gravell
private class SwapVisitor : ExpressionVisitor {
private readonly Expression _from;
private readonly Expression _to;
public SwapVisitor(Expression from, Expression to) {
_from = from;
_to = to;
}
public override Expression Visit(Expression node) {
return node == _from ? _to : base.Visit(node);
}
}
}
var valueSelector = new Expression<Func<MyTable, int>>(o => o.Value);
var intSelector = new Expression<Func<int, bool>>(x => x > 5);
var selector = valueSelector.Chain<MyTable, int, bool>(intSelector);
You can create an overload of Chain that takes a VersatileLambda as the first parameter, and returns a VersatileLambda. Now you're really sizzling along.
You could create a wrapper class. Something like this:
public class FuncExtensionWrap<T>
{
private readonly Expression<Func<T, bool>> exp;
private readonly Func<T, bool> func;
public FuncExtensionWrap(Expression<Func<T, bool>> exp)
{
this.exp = exp;
this.func = exp.Compile();
}
public Expression<Func<T, bool>> AsExp()
{
return this;
}
public Func<T, bool> AsFunc()
{
return this;
}
public static implicit operator Expression<Func<T, bool>>(FuncExtensionWrap<T> w)
{
if (w == null)
return null;
return w.exp;
}
public static implicit operator Func<T, bool>(FuncExtensionWrap<T> w)
{
if (w == null)
return null;
return w.func;
}
}
And then it would be used like this:
static readonly FuncExtensionWrap<int> expWrap = new FuncExtensionWrap<int>(i => i == 2);
// As expression
Expression<Func<int, bool>> exp = expWrap;
Console.WriteLine(exp.Compile()(2));
// As expression (another way)
Console.WriteLine(expWrap.AsExp().Compile()(2));
// As function
Func<int, bool> func = expWrap;
Console.WriteLine(func(1));
// As function(another way)
Console.WriteLine(expWrap.AsFunc()(2));
Here is one workaround. It generates an explicit class for the expression (as the compiler would do under the hood anyway with lambda expressions that require a function closure) instead of just a method, and it compiles the expression in a static constructor so it doesn't have any race conditions that could result in multiple compilations. This workaround still incurs an additional runtime delay as a result of the Compile call which could otherwise be offloaded to build-time, but at least it's guaranteed to run only once using this pattern.
Given a type to be used in the expression:
public class SomeClass
{
public int A { get; set; }
public int? B { get; set; }
}
Build an inner class instead of a method, naming it whatever you would have named the method:
static class SomeClassMeetsConditionName
{
private static Expression<Func<SomeClass,bool>> _expression;
private static Func<SomeClass,bool> _delegate;
static SomeClassMeetsConditionName()
{
_expression = x => (x.A > 3 && !x.B.HasValue) || (x.B.HasValue && x.B.Value > 5);
_delegate = _expression.Compile();
}
public static Expression<Func<SomeClass, bool>> Expression { get { return _expression; } }
public static Func<SomeClass, bool> Delegate { get { return _delegate; } }
}
Then instead of using Where( SomeClassMeetsConditionName() ), you simply pass SomeClassMeetsConditionName followed by either .Delegate or .Expression, depending on the context:
public void Test()
{
IEnumerable<SomeClass> list = GetList();
IQueryable<SomeClass> repo = GetQuery();
var r0 = list.Where( SomeClassMeetsConditionName.Delegate );
var r1 = repo.Where( SomeClassMeetsConditionName.Expression );
}
As an inner class, it could be given an access level just like a method and accessed just like a method, and even collapsed all at once like a method, so if you can stand to look at the class instead of a method, this is a functional workaround. It could even be made into a code template.
Edit
My original question was not very clear, so I'd like to try to rephrase it now. Please let me know if I still miss the mark.
Essentially, I'm trying to return a new string, in which all substrings enclosed by brackets have been replaced by a string from an object in a list. As an abstract example, I want to do something like the following:
public class MyType () : IEquatable <Property>
{
public string id;
public override String ToString()
{
return id;
}
public bool Equals ( MyType other )
{
if ( other is MyType == false )
{
return false;
}
return this.id == other.id;
}
}
List<MyType> listOfCustomTypes = new List<MyType> ();
return Regex.Replace ( originalString, "{(.*?)}", d => listOfCustomTypes.Where ( t => t.id == d.Groups[1].Value ).FirstOrDefault ());
The problem I've run into, or, error, specifically, is Cannot convert lambda expression to delegate type (System.Text.RegularExpressions.MatchEvaluator) because some of the return types in the block are not implicitly convertible to the delegate return type.
I am assuming that it isn't allowed to use return types in delegates, or something, because I can normally access it's properties, or cast to string.
I've probably still managed to jumble my question, so if it helps, my full project can be seen here, and the relevant file for this question is here (specific line is 161).
Original Question
I'm trying to learn how to use delegates and lambdas, and in typical fashion, bit off more than I can chew. In the following code snippet, I define a class, which holds a list of classes. In another function, I'm trying to use a string to find a list item, and get a value from that item.
[XmlRoot ( "Replacers" )]
public class Replacers
{
[XmlElement ( "Property" )]
public List<Property> properties = new List<Property> ();
[XmlIgnore]
public static Replacers replacers;
}
public class Property : IEquatable <Property>
{
[XmlAttribute ( "id" )]
public string id;
[XmlElement ( "Value" )]
public List<Value> propertyValue = new List<Value> ();
public override String ToString()
{
return id;
}
public bool Equals ( Property other )
{
return this.id == other.id && this.propertyValue == other.propertyValue;
}
}
public static class GetVariable
{
public static string FromUser ( string originalString )
{
try
{
//return Regex.Replace ( originalString, "{(.*?)}", m => Replacers.replacers.properties.FindIndex ( m.Groups[1].Value ) );
} catch ( Exception e )
{
return "ERROR: Unable to find '" + Regex.Match ( originalString, "{(.*?)}" ) + "'";
}
}
}
The commented out line above is that I'm trying to figure out. How do I replace anything that matches the pattern {(.*?)} with a value from the list item of the same name.
Thanks for taking the time to consider my question!
TL;DR:
How do I iterate over a list using lambda, where the iteration returns the actual list item? As an example, I want to do something like: Regex.Replace ( input, pattern, m => myList.Where(listItem.identifier == m). I don't feel like I've made my question very clear, so please ask, if you're confused. Thank you!
Consider this example:
var foos = new[] { "aaa", "aba", "aca" };
var bars = foos.Where(f => f.Contains("b")).Select(f => Regex.Replace(f, "b", "d"));
foreach (var bar in bars)
Console.WriteLine(bar);
// Output:
ada
Edit: I'll try to address your comment
A lambda is just a shorthand for a delegate (a typed-method).
You're probably accustomed to types like int, string, double, Animal, etc.
Well, just extend that notion to method signatures.
You can think of any method signature as being a type.
Here's a method that returns a bool, and takes an int as a parameter:
bool A(int i) { ... }
So the signature can be viewed as a type.
A lambda is a shorthand for this. Here's a lambda, that takes an int, and returns a bool, just like the method signature above:
(x) => x % 2 == 0
Linq extension methods (Where(), Select(), etc) all take some delegate type, or lambda expression.
myCollection.Where(x => x % 2 == 0).Where(x => x > 10).Select(x => x * 2);
The beauty is you can keep chaining these extension methods, each one becoming an additional filter if you will.
Select() is special because it's a projection operation (it transforms the items in the collection). You can see it takes this odd parameter here:
Func<string, string> // or something like that, depends on the types in your collection
A Func is kinda like a delegate, but in a more generic sense. It's easy to understand. The first type arguments are the input parameters (think parameters of a method), and the last is the output (think the return type of a method)
Func<in, in, in, out>
Consider this:
// Here's a method signature
bool MyMethod(int a, int b)
// Here's a lambda of the same delegate type
(a, b) => a == b
// Here's a Func being assigned that lambda
Func<int, int, bool> func = (a, b) => a == b;
I have a class MyDummyClass to which I'd like to pass some properties in form of a Lambda expression for a later evaluation. So what I can do something like
public class MyDummyClass<T>
{
public MyDummyClass(Expression<Func<T, object>> property)
{
...
}
...
}
..and then use that class like new MyDummyClass<Person>(x=>x.Name), right?
But then I'd like to pass not only a single property but a list of properties. So I'd write my class like
public class MyDummyClass<T>
{
public MyDummyClass(IEnumerable<Expression<Func<T, object>>> properties)
{
...
}
...
}
and I'd like to use it like new MyDummyClass<Person>(new[] { x=>x.Name, x=>x.Surname }) but unfortunately that doesn't work! Instead I have to write
new MyDummyClass<Person>
(new Expression<Func<Person, object>>[] { x=>x.Name, x=>x.Surname});
But this is a bit awkward to write, isn't it? Of course, using params would work, but this is just a sample out of a more complicated piece of code where using params is not an option.
Does anyone have a better option to come out of this??
Try using params instead:
public MyDummyClass(params Expression<Func<T, object>>[] properties)
Then you should be able to do:
var dummy = new DummyClass<Person>(x => x.Name, x => x.Surname);
You could try:
public class MyDummyClass<T>
{
public MyDummyClass(Expression<Func<T, object>> expression)
{
NewArrayExpression array = expression.Body as NewArrayExpression;
foreach( object obj in ( IEnumerable<object> )( array.Expressions ) )
{
Debug.Write( obj.ToString() );
}
}
}
And then you would call it like this:
MyDummyClass<Person> cls = new MyDummyClass<Person>( item => new[] { item.Name, item.Surname } );
The problem is this won't give you the value of the property because no actual Person instance it specified Doing a ToString on "obj" will give you the name of the property. I don't know if this is what you're after, but it maybe a starting point.
I know there are a few answers on the site on this and i apologize if this is in any way duplicate, but all of the ones I found does not do what I am trying to do.
I am trying to specify method info so I can get the name in a type safe way by not using strings.
So I am trying to extract it with an expression.
Say I want to get the name of a method in this interface:
public interface IMyInteface
{
void DoSomething(string param1, string param2);
}
Currently I can get the name using THIS method:
MemberInfo GetMethodInfo<T>(Expression<Action<T>> expression)
{
return ((MethodCallExpression)expression.Body).Method;
}
I can call the helper method as follows:
var methodInfo = GetMethodInfo<IMyInteface>(x => x.DoSomething(null, null));
Console.WriteLine(methodInfo.Name);
But I am looking for the version that I can get the method name without specifying the parameters (null, null)
like this:
var methodInfo = GetMethodInfo<IMyInteface>(x => x.DoSomething);
But all attempts fail to compile
Is there a way to do this?
x => x.DoSomething
In order to make this compilable I see only two ways:
Go non-generic way and specify it's parameter as Action<string, string>
Specify Action<string, string> as your target delegate type by yourself: GetMethodInfo<IMyInteface>(x => new Action<string,string>(x.DoSomething))
if you are ok to go with second one, which allows you to omit arguments then you can write your GetMethodInfo method as follows:
MemberInfo GetMethodInfo<T>(Expression<Func<T, Delegate>> expression)
{
var unaryExpression = (UnaryExpression) expression.Body;
var methodCallExpression = (MethodCallExpression) unaryExpression.Operand;
var methodInfoExpression = (ConstantExpression) methodCallExpression.Arguments.Last();
var methodInfo = (MemberInfo) methodInfoExpression.Value;
return methodInfo;
}
It works for your interface, but probably some generalization will be required to make this working with any method, that's up to you.
The following is compatible with .NET 4.5:
public static string MethodName(LambdaExpression expression)
{
var unaryExpression = (UnaryExpression)expression.Body;
var methodCallExpression = (MethodCallExpression)unaryExpression.Operand;
var methodCallObject = (ConstantExpression)methodCallExpression.Object;
var methodInfo = (MethodInfo)methodCallObject.Value;
return methodInfo.Name;
}
You can use it with expressions like x => x.DoSomething, however it would require some wrapping into generic methods for different types of methods.
Here is a backwards-compatible version:
private static bool IsNET45 = Type.GetType("System.Reflection.ReflectionContext", false) != null;
public static string MethodName(LambdaExpression expression)
{
var unaryExpression = (UnaryExpression)expression.Body;
var methodCallExpression = (MethodCallExpression)unaryExpression.Operand;
if (IsNET45)
{
var methodCallObject = (ConstantExpression)methodCallExpression.Object;
var methodInfo = (MethodInfo)methodCallObject.Value;
return methodInfo.Name;
}
else
{
var methodInfoExpression = (ConstantExpression)methodCallExpression.Arguments.Last();
var methodInfo = (MemberInfo)methodInfoExpression.Value;
return methodInfo.Name;
}
}
Check this sample code on Ideone.
Note, that Ideone does not have .NET 4.5.
The problem with this is that x.DoSomething represents a method group. And you have to somehow explicitly specify what delegate type do you want to convert that method group into, so that the correct member of the group can be selected. And it doesn't matter if that group contains only one member.
The compiler could infer that you mean that one, but it doesn't do that. (I think it's this way so that your code won't break if you add another overload of that method.)
Snowbear's answer contains good advice on possible solutions.
This is a new answer to an old question, but responds to the "verbose" complaint of the accepted answer. It requires more code, but the result is a syntax like:
MemberInfo info = GetActionInfo<IMyInterface, string, string>(x => x.DoSomething);
or, for methods with a return value
MemberInfo info = GetFuncInfo<IMyInterface, object, string, string>(x => x.DoSomethingWithReturn);
where
object DoSomethingWithReturn(string param1, string param2);
Just like the framework provides Action<> and Func<> delegates up to 16 parameters, you have to have GetActionInfo and GetFuncInfo methods that accept up to 16 parameters (or more, although I'd think refactoring is wise if you have methods with 16 parameters). A lot more code, but an improvement in the syntax.
If you are ok with using the nameof() operator you can use the following approach.
One of the benefits is not having to unwrap an expression tree or supply default values or worry about having a non-null instance of the type with the method.
// As extension method
public static string GetMethodName<T>(this T instance, Func<T, string> nameofMethod) where T : class
{
return nameofMethod(instance);
}
// As static method
public static string GetMethodName<T>(Func<T, string> nameofMethod) where T : class
{
return nameofMethod(default);
}
Usage:
public class Car
{
public void Drive() { }
}
var car = new Car();
string methodName1 = car.GetMethodName(c => nameof(c.Drive));
var nullCar = new Car();
string methodName2 = nullCar.GetMethodName(c => nameof(c.Drive));
string methodName3 = GetMethodName<Car>(c => nameof(c.Drive));
If your application would allow a dependency on Moq (or a similar library), you could do something like this:
class Program
{
static void Main(string[] args)
{
var methodName = GetMethodName<IMyInteface>(x => new Action<string,string>(x.DoSomething));
Console.WriteLine(methodName);
}
static string GetMethodName<T>(Func<T, Delegate> func) where T : class
{
// http://code.google.com/p/moq/
var moq = new Mock<T>();
var del = func.Invoke(moq.Object);
return del.Method.Name;
}
}
public interface IMyInteface
{
void DoSomething(string param1, string param2);
}