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C# properties and "tostring" method

I was trying to understand properties better and I came across this page with this example:
https://www.tutorialspoint.com/csharp/csharp_properties.htm
using System;
namespace tutorialspoint {
class Student {
private string code = "N.A";
private string name = "not known";
private int age = 0;
// Declare a Code property of type string:
public string Code {
get {
return code;
}
set {
code = value;
}
}
// Declare a Name property of type string:
public string Name {
get {
return name;
}
set {
name = value;
}
}
// Declare a Age property of type int:
public int Age {
get {
return age;
}
set {
age = value;
}
}
public override string ToString() {
return "Code = " + Code +", Name = " + Name + ", Age = " + Age;
}
}
class ExampleDemo {
public static void Main() {
// Create a new Student object:
Student s = new Student();
// Setting code, name and the age of the student
s.Code = "001";
s.Name = "Zara";
s.Age = 9;
Console.WriteLine("Student Info: {0}", s);
//let us increase age
s.Age += 1;
Console.WriteLine("Student Info: {0}", s);
Console.ReadKey();
}
}
}
output: Student Info: Code = 001, Name = Zara, Age = 9
I don't understand how the first example is able to output the whole line written in the class "student". In the main method, we are using "s" which is an object created in the class "exampledemo". How is it able to call a method from another class?
I guess it's something related to inheritance and polymorphism (I googled the override keyword) but it seems to me that the two classes are indipendent and not a subclass of the other.
I'm a total beginner at programming and probably quite confused.

s is of type Student (as declared on the first line of Main()). Therefore one can call a method on the object to modify it or print it. When you do s.Name = "Zara"; you are already calling a method on Student to update it (technically, a method and a property are the same, they only differ by syntax).
The line Console.WriteLine("Student Info: {0}", s); is actually the same as Console.WriteLine("Student Info: " + s.ToString());. The compiler allows writing this in a shorter form, but internally the same thing happens.

Let me show a real life example.
You have a sketch of your dream bicycle. Your bicycle exists only in sketch. This is a class.
Then you are going to garage and building your bicycle from the sketch. This process can be called like creation of object
of bicycle at factory from your sketch.
According to your example, class is Student.
You are creating an object by the following line:
Student s = new Student();
Object takes space in memory. How can we read values of objects from memory? By using object reference.
s is an object reference to the newly created object type of Student.
How is it able to call a method from another class?
s is an object reference to the newly created object type of Student. So it can call any public method of this object.
I was trying to understand properties
Properties are evolution of getter and setter methods. Looking for a short & simple example of getters/setters in C#
The compiler generates a pair of get and set methods for a property, plus a private backing field for an auto-implemented property.
Are C# properties actually Methods?

Perform a 'task' via an enum and a field, by checking the 'T' type, and inferring it into the task automatically

I've come across an issue that I cannot solve. I've got an IReadOnlyList of classes that each have a bunch of fields. These fields have names (variable names) identical to a list of enums. Think that for each field that exists, an enum for it with the exact same name also exists (so object helloHi has an equivalent enum something { helloHi }).
What I've attempted to do is compare the two field names. If they are identical, perform a function on them. The problem is that the function needs to infer a T from the variable, and since reflection isn't able to pull that 'T' without some form of cast, it won't proceed.
This is the code:
public class Something() {
[BackgroundTask]
private void load(Overlay param_1, Config param_2) {
Children = new Drawable[] // is the IReadOnlyList
{
SomethingClass(param_1),
AnotherClass(param_2)
}
performTask(this, param_2);
}
}
public class Config {
public void task<U>(SomeEnums se, ValueType<U> value) // do the task
}
public class SomethingClass {
ValueType<double> someDouble = new ValueType<double>();
ValueType<int> someInt = new ValueType<int>();
public SomethingClass(Overlay overlay) //...
}
public enum SomeEnums {
someDouble,
someInt,
}
void performTask(Something the_class, Config the_config) {
// ... for each field within the_class, do (uses reflection)
field => {
foreach (SomeEnums enums in Enum.GetValues(typeof(SomeEnums)))
{
if (field.Name == enums.ToString()) {
the_config.task(enums, field.GetValue(null)); // cant infer 'U' from an 'object'
}
}
}
}
Technically, I could just do the config.task within the class where the types are known and visible, but I'd much prefer to automate it from here, so that it doesn't need 2-3 changes every time a new variable is created.
One of the strategies I am aware of is performing an if check within the performTask like such:
// performTask, field =>, foreach
{
if (field.FieldType == ValueType<double>)
config.task(enums, (ValueType<double>)field.GetValue(null));
} //etc
However, I don't like this method. It would just need to introduce more and more checks if I ever created more ValueType<> and if they aren't already being checked for. Would there be a better way to perform what I want?

As I mentioned in my comment, I can't quite tell what you really want to do. However, here's some code that should help you figure it out.
It uses reflection to get the fields of objects, look at the names of those fields (comparing them to the values/names associated with an enum type) and compare the values. I do a comparison to integer 5, but you could compare to anything (but, it appears that the integer type's implementation of IComparable.CompareTo throws if it's compared to something other than an int, so I check). Since you know the type of everything, this is easy to check (you don't have to compare to a fixed Type, you can use what is returned by GetType()).
I started with some auxiliary types:
public enum SomeEnums {
SomeDouble,
SomeInt,
}
public class Class1 {
public int SomeInt = 5;
public double SomeDouble = 3.14;
}
public class Class2 {
public int SomeInt = 5;
public double SomeDouble = 6.28;
}
and then added this:
public class EnumFields {
public List<object> Objects = new List<object> {
new Class1(),
new Class2(),
};
public void PerformTask () {
var enumVals = Enum.GetNames(typeof(SomeEnums));
foreach (var obj in Objects) {
var objType = obj.GetType();
var fieldInfos = objType.GetFields(System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Public);
//right around here I get lost. You have a list of objects (which has two instances right now),
//What are you comparing, that every field named SomeInt has the same value??
//Anyway, here's some code that should help you
foreach (var fieldInfo in fieldInfos) {
if (enumVals.Contains(fieldInfo.Name)) {
var fieldObj = fieldInfo.GetValue(obj);
var isSame = false;
if (fieldObj.GetType() == typeof(int)) {
var comparable = (IComparable)fieldObj;
var same = comparable.CompareTo(5);
isSame = (same == 0);
}
Debug.WriteLine($"Field: {fieldInfo.Name} of instance of {obj.GetType().Name} (Value: {fieldObj}) is equal to 5:{isSame}");
}
}
}
}
}
When I instantiate an EnumFields object and call PerformTask on it, I see this in the output:
Field: SomeInt of instance of Class1 (Value: 5) is equal to 5:True
Field: SomeDouble of instance of Class1 (Value: 3.14) is equal to 5:False
Field: SomeInt of instance of Class2 (Value: 5) is equal to 5:True
Field: SomeDouble of instance of Class2 (Value: 6.28) is equal to 5:False
This should get you most of the way there. I realize it doesn't answer your question. Had I been able to figure out what you were asking, it probably would have.

Can I change this extension method to remove the "magic string"? [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Finding the Variable Name passed to a Function in C#
The class below contains the field city.
I need to dynamically determine the field's name as it is typed in the class declaration
i.e. I need to get the string "city" from an instance of the object city.
I have tried to do this by examining its Type in DoSomething() but can't find it when examining the contents of the Type in the debugger.
Is it possible?
public class Person
{
public string city = "New York";
public Person()
{
}
public void DoSomething()
{
Type t = city.GetType();
string field_name = t.SomeUnkownFunction();
//would return the string "city" if it existed!
}
}
Some people in their answers below have asked me why I want to do this.
Here's why.
In my real world situation, there is a custom attribute above city.
[MyCustomAttribute("param1", "param2", etc)]
public string city = "New York";
I need this attribute in other code.
To get the attribute, I use reflection.
And in the reflection code I need to type the string "city"
MyCustomAttribute attr;
Type t = typeof(Person);
foreach (FieldInfo field in t.GetFields())
{
if (field.Name == "city")
{
//do stuff when we find the field that has the attribute we need
}
}
Now this isn't type safe.
If I changed the variable "city" to "workCity" in my field declaration in Person this line would fail unless I knew to update the string
if (field.Name == "workCity")
//I have to make this change in another file for this to still work, yuk!
{
}
So I am trying to find some way to pass the string to this code without physically typing it.
Yes, I could declare it as a string constant in Person (or something like that) but that would still be typing it twice.
Phew! That was tough to explain!!
Thanks
Thanks to all who answered this * a lot*. It sent me on a new path to better understand lambda expressions. And it created a new question.

Maybe you need this. Works fine.
I found this here.
static void Main(string[] args)
{
var domain = "matrix";
Check(() => domain);
Console.ReadLine();
}
static void Check<T>(Expression<Func<T>> expr)
{
var body = ((MemberExpression)expr.Body);
Console.WriteLine("Name is: {0}", body.Member.Name);
Console.WriteLine("Value is: {0}", ((FieldInfo)body.Member)
.GetValue(((ConstantExpression)body.Expression).Value));
}
Output will be:
Name is: 'domain'
Value is: 'matrix'

I know this is old question, but I was trying to achieve the same and google sent me here. After many hours I finally found a way. I hope somebody else will find this useful.
There are actually more ways to accomplish this:
static void Main(string[] args)
{
GetName(new { var1 });
GetName2(() => var1);
GetName3(() => var1);
}
static string GetName<T>(T item) where T : class
{
return typeof(T).GetProperties()[0].Name;
}
static string GetName2<T>(Expression<Func<T>> expr)
{
return ((MemberExpression)expr.Body).Member.Name;
}
static string GetName3<T>(Func<T> expr)
{
return expr.Target.GetType().Module.ResolveField(BitConverter.ToInt32(expr.Method.GetMethodBody().GetILAsByteArray(), 2)).Name;
}
The first is fastest. The last 2 are approx 20 times slower than the 1st one.
http://abdullin.com/journal/2008/12/13/how-to-find-out-variable-or-parameter-name-in-c.html

city in this case is an instance of type string. When you call .GetType() you return the actual string type, which has no knowledge at all of your particular city instance.
I'm having a hard time seeing why you can't just type "city" in the code as a string literal here, if that's what you need. Perhaps it would help if you shared what you want to use this value for and in what circumstances you will call your DoSomething() function.
At the moment, my best guess is that what you really want to do is reflect the entire Person class to get a list of the fields in that class:
public void DoSomething()
{
MemberInfo[] members = this.GetType().GetMembers();
// now you can do whatever you want with each of the members,
// including checking their .Name properties.
}
Okay, based on your edit I have some more for you.
You can find the name of fields that are decorated with your attribute at run-time like this:
Type t = typeof(Person);
foreach (MemberInfo member in t.GetMembers()
.Where(m =>
m.GetCustomAttributes(typeof(MyCustomAttribute)).Any() ) )
{
// "member" is a MemberInfo object for a Peson member that is
// decorated with your attribute
}
You can also use binding flags in the first GetMembers() call to limit it to just fields, if you want.

You mentioned "i.e. I need to get the string "city" from an instance of the object city."
Are you looking to get the field name from the value of the field.
For example:If there are 2 Person object one with city "New York" and the other with city "London", are you looking for the function to return "city". Is this what you mean by dynamic?
With your current design you will always need to compare the name of the field from the FieldInfo against a string.
What if you instead decouple this so that you hold the identifier to use for comparison purposes during reflection as part of the attribute.
Something like this:
public enum ReflectionFields
{
CITY = 0,
STATE,
ZIP,
COUNTRY
}
[AttributeUsage(AttributeTargets.Field,AllowMultiple=false)]
public class CustomFieldAttr : Attribute
{
public ReflectionFields Field { get; private set; }
public string MiscInfo { get; private set; }
public CustomFieldAttr(ReflectionFields field, string miscInfo)
{
Field = field;
MiscInfo = miscInfo;
}
}
public class Person
{
[CustomFieldAttr(ReflectionFields.CITY, "This is the primary city")]
public string _city = "New York";
public Person()
{
}
public Person(string city)
{
_city = city;
}
}
public static class AttributeReader<T> where T:class
{
public static void Read(T t)
{
//get all fields which have the "CustomFieldAttribute applied to it"
var fields = t.GetType().GetFields().Where(f => f.GetCustomAttributes(typeof(CustomFieldAttr), true).Length == 1);
foreach (var field in fields)
{
var attr = field.GetCustomAttributes(typeof(CustomFieldAttr), true).First() as CustomFieldAttr;
if (attr.Field == ReflectionFields.CITY)
{
//You have the field and you know its the City,do whatever processing you need.
Console.WriteLine(field.Name);
}
}
}
}
public class Program
{
public static void Main(string[] args)
{
PPerson p1 = new PPerson("NewYork");
PPerson p2 = new PPerson("London");
AttributeReader<PPerson>.Read(p1);
AttributeReader<PPerson>.Read(p2);
}
}
You can now freely rename _city field of Person to something else and your calling code will still work since the code using reflection is trying to identify the field using the ReflectionFields enum value set as part of initialization of the attribute set on the field.

Yes its possible !!!
Try this out...
public string DoSomething(object city)
{
return city.GetType().GetProperty("Name",typeof(string)).GetValue(city,null);
}

Two things here.
Number one, as someone above pointed out, you're getting the Type for string, not for Person. So typeof(Person).GetMembers() will get you the list of members.
Number two, and more importantly, it looks like you're misunderstanding the purpose of attributes. In general attributes are used to mark a member for specific processing or to add additional information. Here you're using the name to indicate what processing you want, and the attribute to specify parameters, which is mixing of metaphors, or something.
Abhijeet's answer is more appropriate, you mark the field as a city field, then do what you like with it. Where I disagree is that I would use different attribute classes, rather than an enumeration.
Something like:
public class MyAttribute : Attribute
{
}
[AttributeUsage(AttributeTargets.Field)]
public class MyCityAttribute : MyAttribute
{
}
[AttributeUsage(AttributeTargets.Field]
public class MyNameAttribute: MyAttribute
{
}
public class Person
{
[MyCity]
public string city = "New York";
[MyCity]
public string workCity = "Chicago";
[MyName]
public string fullName = "John Doe";
public Person()
{
}
public void DoSomething()
{
Type t = typeof(Person);
FieldInfo[] fields = t.GetFields(BindingFlags.Instance | BindingFlags.Public);
foreach (var field in fields)
{
MyAttribute[] attributes = field.GetCustomAttributes(typeof(MyAttribute));
if (attributes.Count > 0)
{
if (attributes[0] is MyCityAttribute)
{
//Dosomething for city
break;
}
if (attributes[0] is MyNameAttribute)
{
//Dosomething for names
break;
}
}
}
}
}
This would allow you to use different parameters for MyCity vs MyName that would make more sense in the context of processing each.
I think with your 'yuk' comment above, you hit the nail on the head. That you would have to change a string constant if you rename your variable is an indicator that you're doing something wrong.

t.GetField("city", BindingFlags.Public | BindingFlags.Instance);
or you can call GetFields() to get all fields

You need to call get type on the class Person. The iterate the fields of the class as in the answer below

This is not possible (I think it actually is but involes several hacks and using lambdas). If you want to store attributes about a Person and be able to get the name of the attribute easily, I suggest using a Dictionary<TKey, TValue> from the System.Collections.Generic namespace.
And you can always make public properties that wrap the dictionary.
public class Person
{
Dictionary<string, string> attributes = new Dictionary<string, string();
public string City
{
get { return attributes["city"]; }
set { attributes["city"] = value; }
}
public Person()
{
City = "New York";
}
}
And you can get a list of all attributes with attributes.Keys.

Have a look at this post as it looks similar to what you're trying to do:
Finding the variable name passed to a function
(especially Konrad Rudolph's answer) Another approach could be to just add "city" as one of the parameters in the attribute and fish that out later.

You are already looping through the collection of FieldInfo objects. Look for your attribute on those and when you find the FieldInfo that contains your attribute, you have the one you want. Then call .Name on it.
system.reflection.fieldinfo.attributes

C# Match User Inputs to Array

I'm writing some code where I have some information about customers stored in an array called members (id, initials). I then ask the user for their id and initials and match the inputs to the stored information from array members. If they match I move on. However I get an error in my coding: "an object reference is required to access non-static field method or property". The error comes from the if statements. Any suggestions on how to correct this issue?
Some background info: I have two classes, one called Customer and one called Menu. The Menu is the main class while Customer is the class I reference from.
This is from my Menu class:
int L = 0;
string I = "";
Customer[] members = new Customer[2];
members[0] = new Customer(3242, "JS");
members[1] = new Customer(7654, "BJ");
Console.Write("\nWhat is your Loyalty ID #: ");
L =Convert.ToInt32(Console.ReadLine());
Console.Write("\nWhat is your first and last name initials: ");
I = Console.ReadLine();
if (L==Customer.GetId())
{
if (I == Customer.GetInitials())
{
Console.WriteLine("It matches");
}
}
else
{
Console.WriteLine("NO match");
}
Console.ReadKey();
}
}
}
This from my Customer class
private int id;
private string initials;
public Customer ()
{
}
public Customer(int id, string initials)
{
SetId(id);
SetInitials(initials);
}
public int GetId()
{
return id;
}
public void SetId(int newId)
{
id = newId;
}
public string GetInitials()
{
return initials;
}
public void SetInitials(string newInitials)
{
initials = newInitials;
}

The error means exactly what it says. You can't access the GetId() function of Customer by calling Customer.GetId() because GetId() only works on an instance of Customer, not directly through the Customer class.
Customer.GetId(); //this doesn't work
Customer myCustomer=new Customer(); myCustomer.GetId(); //this works
To check the user's input against your array of inputs, you need to iterate through the array (or alternatively, use Linq).
I'm going to use a generic list, because there's not really a good reason to use arrays in most cases.
List<Customer> customers=new List<Customer>();
Customers.Add();//call this to add to the customers list.
foreach(var c in customers)
{
if(c.GetId() == inputId)
{
//match!
}
else
{
//not a match
}
}
You can also improve your Customer class by using properties, or auto properties (which doesn't need a backing field). Here's an auto property example:
public string Id {get; set;} // notice there's no backing field?
Using the above auto property syntax would allow you do to this:
var customer = new Customer();
string id = customer.Id; // notice there's no parentheses?
Properties and auto properties allow for a cleaner syntax than having to write Java-style separate getters/setters.

Can I access a class variable with another variable?

i want to do a class constructor that takes a dicionary as parameter and initialize all the class variables that are listed as key in the dictionary, after of course a type conversion:
public class User
{
public int id;
public string username;
public string password;
public string email;
public int mana_fire;
public int mana_water;
public int mana_earth;
public int mana_life;
public int mana_death;
public User ()
{
}
public User(Dictionary<string,string> dataArray){
FieldInfo[] classVariablesInfoList = typeof(User).GetFields();
for(int i = 0; i < classVariablesInfoList.Length; i++)
{
if(dataArray.ContainsKey(classVariablesInfoList[i].Name)){
//missing code here :)
//need something like classVariable= dataArray["classVariablesInfolist[i].name"]; ?
}
}
}
}
but i can't find out how to do this!
Can you please help? :)

You can use the SetValue frunction from reflection:
FieldInfo f = classVariablesInfoList[i];
if (f.ReflectedType == typeof(int))
{
f.SetValue(this, Convert.ToInt32(dataArray[f.Name]));
}
else
{
f.SetValue(this, dataArray[classVariablesInfoList[i].Name]);
}
But it is a really uncommon way to do this with a dictionary. You should considder accessing the fields directly or add parameters to the constructor for any field. And fields should never be public - use properties instead.

The following will work if Convert.ChangeType() is able to handle the conversion. There are a lot of problems waiting to occur, for example handling numbers or dates where the string representation depends on the locale. I would really suggest to use usual typed constructor parameters or standard (de)serialization mechanism if possible. Or at least use a dictionary containing objects instead of strings to get rid of the conversion, again if possible.
public User(Dictionary<String, String> data)
{
var fields = typeof(User).GetFields();
foreach (field in fields)
{
if (data.ContainsKey(field.Name))
{
var value = Convert.ChangeType(data[field.Name], field.MemberType);
field.SetValue(this, value);
}
}
}

I would like to separate your problem into two parts.
1. Applying conversion
The FieldInfo type present a FieldType property that is the actual type of the field, using this Type we can use the non-generic ChangeType method of System.Convert, this method will be able convert some types to others. Luckily it support String to Int.
Usage:
Convert.ChangeType(OLD_VALUE, TARGET_TYPE);
2. Setting the field
The field info class has a SetValue method (FieldInfo.SetValue), it has two parameters, the first one is the current (ie. this) instance (or the instance you wish to change). the second is the new value you wish to set.
Putting it all together
[...]
var fieldInfo = classVariablesInfoList[i];
var name = fieldInfo.Name;
var targetType = fieldInfo.Type;
var value = Convert.ChangeType(dataArray[name], targetType);
classVariablesInfoList[i].SetValue(this, value);