Asked  7 Months ago    Answers:  5   Viewed   22 times

I would like to get the name of a variable or parameter:

For example if I have:

var myInput = "input";

var nameOfVar = GETNAME(myInput); // ==> nameOfVar should be = myInput

void testName([Type?] myInput)
{
   var nameOfParam = GETNAME(myInput); // ==> nameOfParam should be = myInput
}

How can I do it in C#?

 Answers

39

Pre C# 6.0 solution

You can use this to get a name of any provided member:

public static class MemberInfoGetting
{
    public static string GetMemberName<T>(Expression<Func<T>> memberExpression)
    {
        MemberExpression expressionBody = (MemberExpression)memberExpression.Body;
        return expressionBody.Member.Name;
    }
}

To get name of a variable:

string testVariable = "value";
string nameOfTestVariable = MemberInfoGetting.GetMemberName(() => testVariable);

To get name of a parameter:

public class TestClass
{
    public void TestMethod(string param1, string param2)
    {
        string nameOfParam1 = MemberInfoGetting.GetMemberName(() => param1);
    }
}

C# 6.0 and higher solution

You can use the nameof operator for parameters, variables and properties alike:

string testVariable = "value";
string nameOfTestVariable = nameof(testVariable);
Tuesday, June 1, 2021
 
binoculars
answered 7 Months ago
53

Using the python-varname package, you can easily retrieve the name of the variables

https://github.com/pwwang/python-varname

As of v0.6.0, in your case, you can do:

from varname.helpers import Wrapper

foo = Wrapper(dict())

# foo.name == 'foo'
# foo.value == {}
foo.value['bar'] = 2

For list comprehension part, you can do:

n_jobs = Wrapper(<original_value>) 
users = Wrapper(<original_value>) 
queues = Wrapper(<original_value>) 
priorities = Wrapper(<original_value>) 

list_of_dicts = [n_jobs, users, queues, priorities]
columns = [d.name for d in list_of_dicts]
# ['n_jobs', 'users', 'queues', 'priorities']
# REMEMBER that you have to access the <original_value> by d.value

You can also try to retrieve the variable name DIRECTLY:

from varname import nameof

foo = dict()

fooname = nameof(foo)
# fooname == 'foo'

Note that this is working in this case as you expected:

n_jobs = <original_value>
d = n_jobs

nameof(d) # will return d, instead of n_jobs
# nameof only works directly with the variable

I am the author of this package. Please let me know if you have any questions or you can submit issues on Github.

Tuesday, June 1, 2021
 
Teno
answered 7 Months ago
11

Magic Enum header-only library provides static reflection for enums (to string, from string, iteration) for C++17.

#include <magic_enum.hpp>

enum Color { RED = 2, BLUE = 4, GREEN = 8 };

Color color = Color::RED;
auto color_name = magic_enum::enum_name(color);
// color_name -> "RED"

std::string color_name{"GREEN"};
auto color = magic_enum::enum_cast<Color>(color_name)
if (color.has_value()) {
  // color.value() -> Color::GREEN
};

For more examples check home repository https://github.com/Neargye/magic_enum.

Where is the drawback?

This library uses a compiler-specific hack (based on __PRETTY_FUNCTION__ / __FUNCSIG__), which works on Clang >= 5, MSVC >= 15.3 and GCC >= 9.

Enum value must be in range [MAGIC_ENUM_RANGE_MIN, MAGIC_ENUM_RANGE_MAX].

  • By default MAGIC_ENUM_RANGE_MIN = -128, MAGIC_ENUM_RANGE_MAX = 128.

  • If need another range for all enum types by default, redefine the macro MAGIC_ENUM_RANGE_MIN and MAGIC_ENUM_RANGE_MAX.

  • MAGIC_ENUM_RANGE_MIN must be less or equals than 0 and must be greater than INT16_MIN.

  • MAGIC_ENUM_RANGE_MAX must be greater than 0 and must be less than INT16_MAX.

  • If need another range for specific enum type, add specialization enum_range for necessary enum type.

    #include <magic_enum.hpp>
    
    enum number { one = 100, two = 200, three = 300 };
    
    namespace magic_enum {
    template <>
      struct enum_range<number> {
        static constexpr int min = 100;
        static constexpr int max = 300;
    };
    }
    
Tuesday, June 1, 2021
 
Sidarta
answered 7 Months ago
31

I found something that at least begins to answer my own question. The following two links have wmv files from Microsoft that demonstrate using a C# class in unmanaged C++.

This first one uses a COM object and regasm: http://msdn.microsoft.com/en-us/vstudio/bb892741.

This second one uses the features of C++/CLI to wrap the C# class: http://msdn.microsoft.com/en-us/vstudio/bb892742. I have been able to instantiate a c# class from managed code and retrieve a string as in the video. It has been very helpful but it only answers 2/3rds of my question as I want to instantiate a class with a string perimeter into a c# class. As a proof of concept I altered the code presented in the example for the following method, and achieved this goal. Of course I also added a altered the {public string PickDate(string Name)} method to do something with the name string to prove to myself that it worked.

wchar_t * DatePickerClient::pick(std::wstring nme)
{
    IntPtr temp(ref);// system int pointer from a native int
    String ^date;// tracking handle to a string (managed)
    String ^name;// tracking handle to a string (managed)
    name = gcnew String(nme.c_str());
    wchar_t *ret;// pointer to a c++ string
    GCHandle gch;// garbage collector handle
    DatePicker::DatePicker ^obj;// reference the c# object with tracking handle(^)
    gch = static_cast<GCHandle>(temp);// converted from the int pointer 
    obj = static_cast<DatePicker::DatePicker ^>(gch.Target);
    date = obj->PickDate(name);
    ret = new wchar_t[date->Length +1];
    interior_ptr<const wchar_t> p1 = PtrToStringChars(date);// clr pointer that acts like pointer
    pin_ptr<const wchar_t> p2 = p1;// pin the pointer to a location as clr pointers move around in memory but c++ does not know about that.
    wcscpy_s(ret, date->Length +1, p2);
    return ret;
}

Part of my question was: What is better? From what I have read in many many efforts to research the answer is that COM objects are considered easier to use, and using a wrapper instead allows for greater control. In some cases using a wrapper can (but not always) reduce the size of the thunk, as COM objects automatically have a standard size footprint and wrappers are only as big as they need to be.

The thunk (as I have used above) refers to the space time and resources used in between C# and C++ in the case of the COM object, and in between C++/CLI and native C++ in the case of coding-using a C++/CLI Wrapper. So another part of my answer should include a warning that crossing the thunk boundary more than absolutely necessary is bad practice, accessing the thunk boundary inside a loop is not recommended, and that it is possible to set up a wrapper incorrectly so that it double thunks (crosses the boundary twice where only one thunk is called for) without the code seeming to be incorrect to a novice like me.

Two notes about the wmv's. First: some footage is reused in both, don't be fooled. At first they seem the same but they do cover different topics. Second, there are some bonus features such as marshalling that are now a part of the CLI that are not covered in the wmv's.

Edit:

Note there is a consequence for your installs, your c++ wrapper will not be found by the CLR. You will have to either confirm that the c++ application installs in any/every directory that uses it, or add the library (which will then need to be strongly named) to the GAC at install time. This also means that with either case in development environments you will likely have to copy the library to each directory where applications call it.

Wednesday, June 2, 2021
 
DiglettPotato
answered 7 Months ago
76

You can use a method which is taking an Expression<Func<object>> as parameter:

public void WhatDoesTheAnimalSay_WANTED(Expression<Func<object>> expression)
{
    var body = (MemberExpression)expression.Body;
    var variableName = body.Member.Name;

    var func = expression.Compile();
    var variableValue = func();

    MessageBox.Show("The "+ variableName + " says: " + variableValue);
}

Using this approach gives the ability to process a variety of variables (static members, instance members, parameters, local variables etc.), also properties are possible.

Call it like:

WhatDoesTheAnimalSay_WANTED(() => dog)
WhatDoesTheAnimalSay_WANTED(() => Cow)
WhatDoesTheAnimalSay_WANTED(() => Cat)
WhatDoesTheAnimalSay_WANTED(() => kiwi)

Constants are not possible, because the compiler will substitute the constant placeholder by its value, given at compile time:

const string constValue = "Constant Value";

WhatDoesTheAnimalSay_WANTED(() => constValue)

would be transformed to

WhatDoesTheAnimalSay_WANTED(() => "Constant Value")

making the expression.Body of type ConstantExpression, which would yield an exception at runtime.

So you have to be careful what you provide as expression to that method.

Additional Remarks

As you can notice from the comments below, the use of lambda expressions to gather variable names seems controversial.

As @CodeCaster pointed out in one of his comments, there is no officially specified need for the compiler to take the same name of a local variable for the captured member in the anonymous wrapping class.

However, I found this in the remarks of Expression<TDelegate>:

The ability to treat expressions as data structures enables APIs to receive user code in a format that can be inspected, transformed, and processed in a custom manner.

For me this is a sign that the expression trees are exactly designed for purposes like that.

Although it is possible that Microsoft changes this behavior for some reason, there does not seem to be a logical need for doing so. Relying on the principle of least astonishment I'd say it is safe to assume that for an expression from ()=> dog whose Body property is of type MemberExpression, that body.Member.Name resolves to dog.

If it is necessary to have other types for Body also, the method has to be worked out a little bit more. And also it is possible that this will not work in certain circumstances anyways.

Monday, October 11, 2021
 
giraffeslacks
answered 2 Months ago
Only authorized users can answer the question. Please sign in first, or register a free account.
Not the answer you're looking for? Browse other questions tagged :
 
Share