Develop Reference

C# - Can I define a property's "set" method with extra actions without creating a private var for the value?

Usually, I see things implemented like this:
private string _privateProperty;
public string PublicProperty
{
get { return _privateProperty; }
set { DoSomething(); _privateProperty = value; }
}
But is it really necessary to have this extra var?
Can't I do something similar to { get; set; } containing the extra method?
//pseudo code
public string PublicProperty { get; set {DoSomething(); <set_value>; }

From the documentation:
In C# 3.0 and later, auto-implemented properties make
property-declaration more concise when no additional logic is required
in the property accessors.
So for the time being you can’t.

Why not explicitly initialized readonly autoimplemented property is valid in c# 6?

Update 1
It seems like either my English is terribly awful, or people just don't give a sh... to understand what I'm asking about or simply look at the title of the post.
C#5 specification clearly states:
Because the backing field is inaccessible, it can be read and written
only through the property accessors, even within the containing type.
This means that automatically implemented read-only or write-only
properties do not make sense, and are disallowed.
public string MyProperty {get;} has no sense, yet it costs nothing for compiler to emit getter not even warring about lacking setter. Backing field will be initialized with a default value. What does it mean? It means that designers spent some effort to implement a verification, to introduce functionality that could be left out.
Let's now consider C#6:
In C#6 the initialization of auto-implemented properties was introduced.
public string FirstName { get; set; } = "Jane";
or
public string FirstName { get; } = "Jane";
In the latter case property can be set in a constructor as well:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
}
public Program()
{
ImagePath = "";
}
}
But only in constructor of the class where property was declared. Derived classes cannot set property's value.
Now ask yourself what this property means, if it was not initialized in constructor:
property string My {get;}
This is a 100% equivalent of C#5 prohibited property. It has no sense.
But such declaration being invalid in C#5 became valid in C#6. However semantics didn't change at all: this property is useless without explicit initialization.
That's why I am asking:
Why not explicitly initialized readonly auto-implemented property is valid in c# 6?
What I expect to see as an answer:
Either debunking of my initial assumptions about changes in C#6
Or the explanation of how and why compiler designers changed their mind
about what makes sense, and what does not.
I find the answer It's by design to be completely irrelevant. It is just a fact. I look for reasons. I don't believe compiler designers decide on changes in behavior of compiler with just tossing the coin.
This is an example of good answer.
Original question
In VS2015 this code is compiled without errors:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
Console.WriteLine("Hello World");
}
}
However, in VS2013 I get error:
Compilation error (line 5, col 28): 'Program.ImagePath.get' must
declare a body because it is not marked abstract or extern.
Automatically implemented properties must define both get and set
accessors.
I know about initializable auto implemented properties, and in case of VS2015 field gets default value, that is null here. But then it's interesting to know why this snippet was invalid in C# 5?
Initializable auto-implemented readonly property left without explicit initialization seems to me a bit ODD. It is likely a mistake rather than intention. I'd personally prefer compiler to require explicit initialization in this case:
public string ImagePath { get; } = default(string);
Ok, I know that such property can be also assigned in constructor:
public class Program
{
public string ImagePath { get; }
public static void Main()
{
}
public Program()
{
ImagePath = "";
DoIt();
}
public void DoIt()
{
//ImagePath = "do it";
}
}
public class My : Program
{
public My()
{
//ImagePath = "asdasd";
}
}
But if compiler can check that local variable is not initialized, the same is possible for the property.
So why is it as it is?

The compiler is telling you that automatic properties must have both accessors defined. For example, you could fix the error with
public string ImagePath { get; private set; }
assuming that you do not intend the property to be settable outside the class.
As to why you have to declare a setter or manually implement the property -- well, what good would be a property that you can read from, but will always return the default value of its type since there is no way to set it? Conversely, what good would be a property you can write to but can neither read from nor hook into its setter?
C# 6.0 gives you the option of having write-once, read many auto-properties; this is a huge difference as the value can be arbitrarily chosen, allowing you convenient syntax for properties with immutable values.

I have no idea why your question have been down voted. This is interesting observation but please keep in mind that it is not a breaking change - it is just 'new functionality' that is 'leftover' of other functionality - initialization of auto-implemented properties.
That means it had no sense previously, but now it has.
Moreover, I think it has always had sense. E.g. when you have some base class or interface, for example
interface IPerson
{
int Age { get; }
}
Some day you may want to implement null-object pattern where the age is irrelevant. In c#5 you have to write public int Age { get { return 0; } }, while in c#6 you can simply do public int Age { get; } or even transform interface to abstract class changing only its definition from interface to abstract class.

Read-Only Property in C# 6.0

Microsoft introduce a new syntax in C#6 that let you set your property to read-only as below:
public class Animal
{
public string MostDangerous { get; } = "Mosquito";
}
I am wondering what is the added value of such approach.
What is the difference by just writing:
public class Animal
{
public const string MostDangerous = "Mosquito";
}
or even:
public class Animal
{
public string MostDangerous
{
get
{
return "Mosquito";
}
}
}

Your example is using string constants which can't show all the possibilities. Look at this snippet:
class Foo
{
public DateTime Created { get; } = DateTime.Now; // construction timestamp
public int X { get; }
public Foo(int n)
{
X = n; // writeable in constructor only
}
}
Read only properties are per-instance and can be set from the constructor. Very different from a const field whose value must be determined at compile time. The property initializer is a separate feature and follows the rules and limitations of field initializers.

The newer syntax is an effort in reducing the verbosity of C#. It's just syntactic sugar. The IL generated is similar to an auto property with a getter and a backing store.

This improvement to C# was taken directly from VB, and removes the need to implement the backing field and constructor initializer:
Public ReadOnly dateStamp As DateTime = Datetime.Now

How to make a property required in c#?

I have requirement in a custom class where I want to make one of my properties required.
How can I make the following property required?
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}

If you mean "the user must specify a value", then force it via the constructor:
public YourType(string documentType) {
DocumentType = documentType; // TODO validation; can it be null? blank?
}
public string DocumentType {get;private set;}
Now you can't create an instance without specifying the document type, and it can't be removed after that time. You could also allow the set but validate:
public YourType(string documentType) {
DocumentType = documentType;
}
private string documentType;
public string DocumentType {
get { return documentType; }
set {
// TODO: validate
documentType = value;
}
}

.NET 7 or newer
Syntax
public class MyClass
{
public required string Name { get; init; }
}
new MyClass(); // illegal
new MyClass { Name = "Me" }; // works fine
Remarks
The required properties must declare a setter (either init or set).
Access modifiers on properties or setters cannot be less visible than their containing type, as they would make impossible to initialize the class in some cases.
public class MyClass
{
internal required string Name { get; set; } // illegal
}
Documentation
Official documentation here
Feature demo here
.NET 6 or older
See this answer

If you mean you want it always to have been given a value by the client code, then your best bet is to require it as a parameter in the constructor:
class SomeClass
{
private string _documentType;
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
public SomeClass(string documentType)
{
DocumentType = documentType;
}
}
You can do your validation – if you need it – either in the property's set accessor body or in the constructor.

With the release of .NET 7 and C# 11 in November 2022 you can now use the required modifier this way:
public class Person
{
public Person() { }
[SetsRequiredMembers]
public Person(string firstName) => FirstName = firstName;
public required string FirstName { get; init; }
public int Age { get; set; }
}
And when you don't have the required properties it will throw an error when you try to initialize an object.
For more information refer to:
https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11#required-members
https://learn.microsoft.com/en-us/dotnet/csharp/properties#init-only

Add a required attribute to the property
Required(ErrorMessage = "DocumentTypeis required.")]
public string DocumentType
{
get
{
return _documentType;
}
set
{
_documentType = value;
}
}
For custom attribute detail Click Here

I used an other solution, not exactly what you want, but worked for me fine because I declare the object first and based on specific situation I have different values. I didnt want to use the constructor because I then had to use dummy data.
My solution was to create Private Sets on the class (public get) and you can only set the values on the object by methods. For example:
public void SetObject(string mandatory, string mandatory2, string optional = "", string optional2 = "")

This one liner works in C# 9:
public record Document(string DocumentType);
new Document(); // compiler error
new Document("csv"); // correct way to construct with required parameter
This explains how it works. In the above code, Document is the name of the class or "record". That first line of code actually defines an entire class. In addition to this solution essentially making a required DocumentType property (required by an auto implemented constructor), because it uses records, there are additional implications. So this may not always be an appropriate solution, and the C# 11 required keyword will still come in handy at times. Just using record types doesn't automatically make properties required. The above code is a special syntax way of using records that essentially has this effect as well as making the property init only and causes a deconstructor to be automatically implemented.
A better example would be using an int property instead of a string since a string could still be empty. Unfortunately I don't know of any good way to do extra validation within the record to make sure the string is not empty or an int is in range, etc. You would have to go deeper down the TOP (type driven development) rabbit hole, which may not be a bad thing. You could create your own type that doesn't allow empty strings or integers outside your accepted range. Unfortunately such an approach would lead to runtime discovery of invalid input instead of compile time. There might be a better way using static analysis and metadata, but I've been away from C# for too long to know anything about that.

Accessing members in your own class: use (auto)properties or not?

I've created this "question" as a community-wiki, because there is no right or wrong answer. I only would like to know how the community feels about this specific issue.
When you have a class with instance variables, and you also created properties that are simply getters and setters for these instance variables, should you use the properties inside your own class, or should you always use the instance variable?
Having auto-properties in C# 3.0 made this an even harder decision.
Using properties:
public class MyClass
{
private string _name;
// could be an auto-property of-course
public string Name { get { return _name; } set { _name = value; } }
public void Action()
{
string localVar = Name;
// ...
Name = "someValue";
// ...
}
}
Using instance variables:
public class MyClass
{
private string _name;
public string Name { get { return _name; } set { _name = value; } }
public void Action()
{
string localVar = _name;
// ...
_name = "someValue";
// ...
}
}
(for those who hate member prefixes, I apologize)
Personally, I always use the latter (instance variables), because I feel that properties should only be used by other classes, not yourself. That's why I mostly stay away from auto-properties as well.
Of course, things change when the property setter (or getter) does a little more than just wrapping the instance variable.
Are there compelling reasons to pick one or the other?

I always use instance variables as well. The reason is because properties might be doing stuff like validating arguments (like in a setter) for not null or not empty. If you're using the variable inside your class code, there's no need to go through the extra overhead of those checks (assuming you know the variable value is valid). The properties could be doing other things as well (logging, for example), that are important for the public API, but not for internal usage, so again, it's better to avoid the overhead and just use the instance variable in my opinion.

I think it becomes more difficult to change the internal implementation if the code uses its own public interface.
Difficult to explain but consider these expressions:
mTotalPrice = mPrice * mQuantity;
mTotalPrice = Price * Quantity;
What to do in the second expression if I need to change the internals to express all prices in € instead of $ (without affecting the public interface which still uses $)?
One solution is to make the expression more complex by adding the opposite of the change in the property.
mTotalPrice = Price / Rate * Quantity
The other solution is to start to use the private field instead.
mTotalPrice = mPrice * Quantity
In the end you get a mix of private and public use. The only way to get consistent use is to always use the private field.

I don't like prefixing members either, but actually I find I can write something like this accidently and not spot it until run time. Which kinda tempts me to avoid using properties where they're not necessary... but I still do, currently!
Public String MyString
{
{ get { return this.MyString; } } //<== Stack Overflow
{ set { this.myString = value; } }
}
private String myString;

I think that there is no difference between these two approaches.
Auto-implemented properties is just a quick way to access private members which are created any way.
Example from MSDN:
class Customer
{
// Auto-Impl Properties for trivial get and set
public double TotalPurchases { get; set; }
public string Name { get; set; }
public int CustomerID { get; set; }
// Constructor
public Customer(double purchases, string name, int ID)
{
TotalPurchases = purchases;
Name = name;
CustomerID = ID;
}
// Methods
public string GetContactInfo() {return "ContactInfo";}
public string GetTransactionHistory() {return "History";}
// .. Additional methods, events, etc.
}

99% of the time I use the property rather then the instance variable. In the past, I've worked with a lot of code that used the instance variable and when there was a bug associated with that variable, I had to put a breakpoint on every line of code that referenced it.
I decided to use properties instead, either public or private, to wrap around the instance variable. Doing this means that I only have to put a breakpoint in the getter/setter of the property if I need to debug an issue with the instance variable, rather then having (potentially) a lot of breakpoints scattered all over the code.