C# Abstract Property

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Syntax and Implementation

public abstract class BaseClass
{
    // Abstract property with both get and set accessors
    public abstract string Identifier { get; set; }

    // Abstract read-only property
    public abstract int Threshold { get; }

    // Abstract property with restricted accessor visibility
    public abstract double Factor { get; protected set; }
}

public interface IConfigurable
{
    // Explicitly abstract property in an interface (C# 8.0+)
    abstract string ConfigKey { get; }
}

public class DerivedClass : BaseClass, IConfigurable
{
    private string _identifier;

    // Overriding a read-write property (can use backing fields or auto-properties)
    public override string Identifier 
    { 
        get => _identifier; 
        set => _identifier = value; 
    }

    // Overriding a read-only property
    public override int Threshold => 100;

    // Overriding a property with restricted accessor visibility
    public override double Factor { get; protected set; }

    // Implementing the interface abstract property
    public string ConfigKey => "DefaultKey";
}

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Core Mechanics and Compiler Rules

• Declaration Context: Abstract properties can be declared inside classes explicitly marked with the abstract modifier. Since C# 8.0 (Default Interface Methods), properties inside interfaces can also be explicitly marked with the abstract keyword.
• No Body: The base declaration cannot contain any logic. Writing get { return 0; } on an abstract property will result in a compiler error.
• Mandatory Implementation: Any concrete (non-abstract) class inheriting from the base type is required by the compiler to implement the abstract property. For abstract base classes, this requires the override keyword.
• Signature Matching: The overriding property in the derived class must exactly match the type, name, and access modifiers of the abstract property.
• Accessor Parity: The derived class must implement exactly the accessors defined in the base abstract property.
  • If the abstract property defines only a get accessor, the derived class must provide only a get accessor. Attempting to add a set accessor to the overridden property results in compiler error CS0546 (cannot override because it does not have an overridable set accessor).
  • If the abstract property defines both get and set, the derived class must implement both.
• Accessor Accessibility: If an abstract property restricts the visibility of a specific accessor (e.g., public abstract int Value { get; protected set; }), the overriding property must enforce the exact same visibility modifier on that specific accessor.