Develop Reference

Pass a lambda expression as a parameter, then execute that lambda with a parameter

Say I have the following (incorrect) code;
public void Foo()
{
bool retVal = Bar(x => x.Any(y => y.Contains(z)); // Where z is "my variable" (below)
}
public bool Bar(Func<List<MyObject>, bool> pFunc)
{
return pFunc("a variable");
}
How do I pass the lambda expression, written in the call to Bar, so that it is executed in Bar, using an additional variable that only exists in Bar()?
The code does not have to remain this simple.

You can have two inputs in a lambda: (x,z)
public void Foo()
{
bool retVal = Bar((x,z) => x.Any(y => y.Contains(z))); // Where z is "my variable" (below)
}
public bool Bar(List<MyObject> list, Func<List<MyObject>, string, bool> pFunc)
{
return pFunc(list, "a variable");
}
Which means you have to update the Func signature to Func<List<MyObject>, string, bool> as well to reflect (x,z)=>bool
I also updated Bar signature to give the input list because it was missing. Maybe it is a local variable or class member. Anyway you should be able to adapt your code starting from here

Func to get the property instead of just the value

Is it possible to input an object and a second parameter to a method so that the second parameter can be used (without using strings and reflection) to get the a property of the object and use the property to both read and write the value?
I've written two methods below that I can use but each one has drawbacks. The first DoStuff is typesafe ("good") but it needs three parameters ("bad"). The second needs two parameters (optimal), but it is not typesafe ("bad") as it relies on a string to specify the property. Maybe there is some solution using Expression that I have not thought of?
Background: The usecase is that I want to be able to "extend" the value of any object (in my case I have lists of objects from several object reposites and these objects may have serveral properties containing userids as strings. another repository conatins the users and I want add info about the user to the strings in the previous repositories)
public class Foo
{
public string Bar {get; set;}
}
void Main()
{
var x = new Foo { Bar = "hello" };
Console.WriteLine(x.Bar); // prints "hello"
// the second paramter (Func<..>) gets the value, the third parameter Action<...>) sets the value
DoStuff(x, y => y.Bar, (z, val) => z.Bar = val);
Console.WriteLine(x.Bar); // prints "hello goodbye"
// just one parameter to get the property, but I would like this to be more type safe, preferably a Func
DoStuff2(x, nameof(x.Bar));
Console.WriteLine(x.Bar); // prints "hello goodbye again"
}
public void DoStuff<T>(
T obj,
Func<T, string> getProp,
Action<T, string> setProp)
{
var x = getProp(obj);
setProp(obj, x + " goodbye");
}
public void DoStuff2<T>(
T obj,
string propName)
{
var propInfo = typeof(T).GetProperty(propName);
var currValue = propInfo.GetValue(obj) as string;
propInfo.SetValue(obj, currValue + " again");
}

Well i did something like that awhile ago. here is an example:
public void SetValue<T, TP>(T obj, Expression<Func<T, TP>> action, TP value) where T : class
{
var member = action.Body is UnaryExpression
? ((MemberExpression)((UnaryExpression)action.Body).Operand)
: (action.Body is MethodCallExpression
? ((MemberExpression)((MethodCallExpression)action.Body).Object)
: (MemberExpression)action.Body);
var key = member?.Member.Name;
typeof(T).GetProperty(key).SetValue(obj, value);
}
You call it like this.
SetValue<User>(x=> x.UserName, "Admin");

Func<> with lambda expression

I have found following part of code in some examples while learning Func<> syntax:
public static class Lambda
{
public static int MyFunc(Func<string, int> func)
{
//some logic
return 0;
}
}
And sample call :
var getInt = Lambda.MyFunc((url) => { Console.WriteLine(url); return 0; }
And My Question :
Why passing above func as lambda expression with this (url) is allowed if value is never assigned ( or maybe is ?)?
What is the point of passing Func like this ?
Edit :
To clarify my question . I was only wondering about this sample call - why passing string as argument like above (using lambda (url) => {} ) is not forbidden by compiler if the value can not be initiated. Is there any example that can be useful with passing string like above ?

url is the name of the parameter for the lambda expression. It's like writing a method like this:
public static int Foo(string url)
{
Console.WriteLine(url);
return 0;
}
Then creating a delegate from it:
Func<string, int> func = Foo;
Now in order to call the delegate, you need to provide it a string - and that then becomes the value of the parameter, just like if you called the method normally:
int result = func("some url");

Setting parameter values of anonymous function in C#

Lets say i have the following code
private Func<T> _method;
public void SetExecutableMethod<T>(Func<T> methodParam)
{
_method = methodParam;
}
public T ExecuteMethod(object[] parameterValues)
{
//get the number of parameters _method has;
var methodCallExpression = _method.Body as MethodCallExpression;
var method = methodCallExpression.Method;
ParameterInfo[] methodParams = method.GetParameters();
//So i now have a list of parameters for the method call,
//How can i update the parameter values for each of these?
for (int i = 0; i < parameters.Count(); i++ )
{
methodParams[i] = ???''
}
return _method.Compile()();
}
public void InitAndTest()
{
SetExecutableMethod( () => _service.SomeMethod1("param1 placeholder", "param2 placeholder") );
T result1 = ExecuteMethod(new object[]{"Test1", "Test2"});
T result2 = ExecuteMethod(new object[]{"Test3", "Test4"}););
}
In the above code, i want to set a private variable to some Func that points to an anonymoust function and never have to set it again.
I then would like to be able to call ExecuteMethod(...) with different parameters. This method should update the parameter values of the variable _method and then invoke the method.
I can read the number of parameters and their values fine, i just am not sure how to set the values for those parameter? Any thoughts on this?

This is not the way to do it. Right now, your _method field is a delegate of type Func<T>, and you expect that its body contains yet another method which is actually executed. That is a lot to expect from your callers. I would forget about this approach and look for something different.
One way would be to supply a method which takes an array of objects as its parameter (Func<object[], T>), and then invoke it directly with appropriate parameters (but never a method in its body). Even this is less common for a strongly typed language like C# since you lose all type safety (but then again, you do want to be pretty flexible with this framework you are designing).
The other way would be to get a MethodInfo instance, and then use its Invoke method. In a way, this might even express your intents better, because it will be obvious that the executable method is capable of virtually anything.
Next, you could use generics to get some type safety, and require that all your input parameters are wrapped inside a single parameter class. In that case, you might have a strongly typed Func<Tparam, Tresult> method, and your Execute method would accept a Tparam instance as its parameter. This would void the need for any reflection.
[Edit]
As I wrote, I would try to avoid reflection. Since you wrote you basically need a cache of method results, a simple approach might be something like:
Create a wrapper for your list of parameters so that you can compare them "by value". I added an example class, but you might even want to allow passing an IEqualityComparer explicitly, so that you don't have to override Equals for each partial parameter.
// implements `IEquatable` for a list of parameters
class Parameters : IEquatable<Parameters>
{
private readonly object[] _parameters;
public Parameters(object[] parms)
{
_parameters = parms;
}
#region IEquatable<Parameters> Members
public bool Equals(Parameters other)
{
if (other == null)
return false;
if (_parameters.Length != other._parameters.Length)
return false;
// check each parameter to see if it's equal
// ...
}
public override bool Equals(object obj)
{
return Equals(obj as Parameters);
}
public override int GetHashCode()
{ ... }
#endregion
}
Create a cache for a single service. Using the wrapper class above, it should simply check if a cached result exists:
// contains cached results for a single service
class CachedCallInfo
{
private readonly Func<object[], object> _method;
private readonly Dictionary<Parameters, object> _cache
= new Dictionary<Parameters, object>();
public CachedCallInfo(Func<object[], object> method)
{
_method = method;
}
public T GetResult<T>(params object[] parameters)
{
// use out Parameters class to ensure comparison
// by value
var key = new Parameters(parameters);
object result = null;
// result exists?
if (!_cache.TryGetValue(key, out result))
{
// do the actual service call
result = _method(parameters);
// add to cache
_cache.Add(key, result);
}
return (T)result;
}
}
Create the final class which will reference services by name:
public class ServiceCache
{
private readonly Dictionary<string, CachedCallInfo> _services =
new Dictionary<string, CachedCallInfo>();
public void RegisterService(string name, Func<object[], object> method)
{
_services[name] = new CachedCallInfo(method);
}
// "params" keyword is used to simplify method calls
public T GetResult<T>(string serviceName, params object[] parameters)
{
return _services[serviceName].GetResult<T>(parameters);
}
}
Your cache setup will then look like this:
serviceCache.RegisterService("ServiceA", #params => DoSomething(#params));
serviceCache.RegisterService("ServiceB", #params => SomethingElse(#params));
And you would simply call it like this:
var result = serviceCache.GetResult("ServiceA", paramA, paramB, paramC);

Not sure why this is useful, but here goes:
public class SomeCrazyClass<T>
{
private Expression<Func<T>> _method;
public void SetExecutableMethod(Expression<Func<T>> methodParam)
{
_method = methodParam;
}
public object ExecuteMethod(SomeService someService, object[] parameterValues)
{
var methodCallExpression = _method.Body as MethodCallExpression;
var method = methodCallExpression.Method;
var methodCall = Expression.Call(Expression.Constant(someService), method,
parameterValues.Select(Expression.Constant));
return Expression.Lambda(methodCall).Compile().DynamicInvoke();
}
}
Call it like so:
public static void InitAndTest()
{
var something = new SomeCrazyClass<int>(); //or whatever type your method returns
var _service = new SomeService();
something.SetExecutableMethod(() => _service.SomeMethod1("param1 placeholder", "param2 placeholder"));
var result1 = something.ExecuteMethod(_service,new object[] {"Test1", "Test2"});
var result2 = something.ExecuteMethod(_service, new object[] {"Test3", "Test4"});
}

Personally, I think you're going WAY overboard, unless there is an overriding architecture need to deal with the lambda as an expression tree. But, I digress.
Instead of working with the reflective elements (which are basically for description only in terms of an expression tree), look at the Arguments member of your MethodCallExpression. It will contain several ContantExpression objects, which you can replace with your own ConstantExpressions containing the string values you want to pass in. However, Expressions are read-only; you have to rebuild an equivalent tree for this call.
public class FuncManipulator<T>
{
private Func<T> _method;
public void SetExecutableMethod(Func<T> methodParam)
{
_method = methodParam;
}
//you forgot the "params" keyword
public T ExecuteMethod(params object[] parameterValues)
{
//get the number of parameters _method has;
var methodCallExpression = _method.Body as MethodCallExpression;
var arguments = methodCallExpression.Arguments;
var newArguments = new List<Expression>();
for (int i = 0; i < arguments.Count(); i++ )
{
newArguments.Add(Expression.Constant(parameterValues[i]));
}
//"Clone" the expression, specifying the new parameters instead of the old.
var newMethodExpression = Expression.Call(methodCallExpression.Object,
methodCallExpression.Method,
newArguments)
return newMethodExpression.Compile()();
}
}
...
public void InitAndTest()
{
SetExecutableMethod( () => _service.SomeMethod1("param1 placeholder", "param2 placeholder") );
T result1 = ExecuteMethod("Test1", "Test2");
T result2 = ExecuteMethod("Test3", "Test4");
T result3 = ExecuteMethod("Test6", "Test5");
}
This will work as long as the expression tree can find the Func referred to by MethodCallExpression.method within the current instance.
However, I think there's a much simpler way:
public class FuncManipulator<T>
{
private Func<T> _method;
public void SetExecutableMethod(Func<T> methodParam)
{
_method = methodParam;
}
//you must pass the actual array; we are creating a closure reference that will live
//as long as the delegate
public void SetMethodParams(object[] param)
{
_param = param;
}
public T ExecuteMethod(params object[] passedParam)
{
//We have to re-initialize _param based on passedParam
//instead of simply reassigning the reference, because the lambda
//requires we don't change the reference.
for(int i=0; i<_param.Length; i++)
_param[i] = passedParam.Length <= i ? null : passedParam[i];
//notice we don't pass _param; the lambda already knows about it
//via the reference set up when declaring the lambda.
return _method();
}
}
...
public void InitAndTest()
{
//this is an "external closure" we must keep in memory
object[] param = new object[2];
SetExecutableMethod( () => _service.SomeMethod1(param[0], param[1]) );
//We do so by passing the reference to our object
SetMethodParams(param);
//now, don't ever reassign the entire array.
//the ExecuteMethod function will replace indices without redefining the array.
T result1 = ExecuteMethod("Test1", "Test2");
T result2 = ExecuteMethod("Test3", "Test4");
T result3 = ExecuteMethod("Test6", "Test5");
}

Passing a Function (with parameters) as a parameter?

I want to create a generic to which I can pass a function as a parameter, however this function may include parameters itself so...
int foo = GetCachedValue("LastFoo", methodToGetFoo)
Such that:
protected int methodToGetFoo(DateTime today)
{ return 2; // example only }
Essentially I want to have a method that will check the cache for a value, otherwise will generate the value based on the passed in method.
Thoughts?

It sounds like you want a Func<T>:
T GetCachedValue<T>(string key, Func<T> method) {
T value;
if(!cache.TryGetValue(key, out value)) {
value = method();
cache[key] = value;
}
return value;
}
The caller can then wrap this in many ways; for simple functions:
int i = GetCachedValue("Foo", GetNextValue);
...
int GetNextValue() {...}
or where arguments are involved, a closure:
var bar = ...
int i = GetCachedValue("Foo", () => GetNextValue(bar));

Use System.Action and a lambda expression (anonymous method). For example:
public void myMethod(int integer) {
// Do something
}
public void passFunction(System.Action methodWithParameters) {
// Invoke
methodWithParameters();
}
// ...
// Pass anonymous method using lambda expression
passFunction(() => myMethod(1234));

You can create your own delegate, but in C# 3.0 you may find it more convenient to use the built-in Func<T> delegate family to solve this problem. Example:
public int GetCachedValue(string p1, int p2,
Func<DateTime, int> getCachedValue)
{
// do some stuff in here
// you can call getCachedValue like any normal function from within here
}
This method will take three arguments: a string, an int, and a function that takes a DateTime and returns an int. For example:
int foo = GetCachedValue("blah", 5, methodToGetFoo); // using your method
int bar = GetCachedValue("fuzz", 1, d => d.TotalDays); // using a lambda
Different Func<T, U, V...> etc. types exist in the framework to accommodate methods with different amounts of arguments.

Create a delegate for the method methodToGetFoo
public delegate object GenerateValue(params p);
public event GenerateValue OnGenerateValue;
Define GetCachedValue to use the delegate
int GetCachedValue(string key, GenerateValue functionToCall);
Then in the implementation of OnGenerateValue you can check the param's.

Here is something simple I started that can be taken a bit further (as I did for a commercial project).
In my case this was to cache web service calls, and was used something like:
WebService ws = new WebService();
var result = ws.Call( x => x.Foo("bar", 1)); // x is the ws instance