Done.

Classes/BaseTypes/NotifyBase.cs

@@ -0,0 +1,66 @@
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+namespace Los.Santos.Dope.Wars.Classes.BaseTypes;
+
+/// <summary>
+///	The <see cref="NotifyBase"/> class.
+/// <remarks>
+///	The <see cref="NotifyBase"/> class implements the following interfaces:
+/// <list type="bullet">
+///		<item>The members of the <see cref="INotifyPropertyChanged"/> interface.</item>
+///		<item>The members of the <see cref="INotifyPropertyChanging"/> interface.</item>
+/// </list>
+/// </remarks>
+/// </summary>
+public abstract class NotifyBase : INotifyPropertyChanged, INotifyPropertyChanging
+{
+	/// <summary>
+	/// Sets a new value for a property and notifies about the change.
+	/// </summary>
+	/// <remarks>
+	/// Will throw an exception of type <see cref="ArgumentNullException"/> if <paramref name="propertyName"/> is <see langword="null"/>.
+	/// </remarks>
+	/// <typeparam name="T"></typeparam>
+	/// <param name="field">The referenced field.</param>
+	/// <param name="newValue">The new value for the property.</param>
+	/// <param name="propertyName">The property name.</param>
+	/// <exception cref="ArgumentNullException"></exception>
+	protected void SetProperty<T>(ref T field, T newValue, [CallerMemberName] string? propertyName = null)
+	{
+		if (propertyName is null)
+			throw new ArgumentNullException(nameof(propertyName));
+
+		if (!Equals(field, newValue))
+		{
+			RaisePropertyChanging(propertyName);
+			field = newValue;
+			RaisePropertyChanged(propertyName);
+		}
+	}
+
+	/// <inheritdoc/>
+	public event PropertyChangedEventHandler? PropertyChanged;
+	/// <inheritdoc/>
+	public event PropertyChangingEventHandler? PropertyChanging;
+
+	/// <summary>
+	/// The method is used to raise the 'property changed' event.
+	/// </summary>
+	/// <remarks>
+	/// The calling member's name will be used as the parameter.
+	/// </remarks>
+	/// <param name="propertyName">The name of the property, can be <see langword="null"/>.</param>
+	protected virtual void RaisePropertyChanged([CallerMemberName] string? propertyName = null) =>
+		PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+
+	/// <summary>
+	/// The method is used to raise the 'property changing' event.
+	/// </summary>
+	/// <remarks>
+	/// The calling member's name will be used as the parameter.
+	/// </remarks>
+	/// <param name="propertyName">The name of the property, can be <see langword="null"/>.</param>
+	protected virtual void RaisePropertyChanging([CallerMemberName] string? propertyName = null) =>
+		PropertyChanging?.Invoke(this, new PropertyChangingEventArgs(propertyName));
+}

Classes/Drug.cs

@@ -1,49 +1,22 @@
-using System.Xml.Serialization;
+using Los.Santos.Dope.Wars.Classes.BaseTypes;
+using System.ComponentModel;
+using System.Xml.Serialization;
 
 namespace Los.Santos.Dope.Wars.Classes;
 
 /// <summary>
 /// The <see cref="Drug"/> class is the root element for the drug class.
 /// </summary>
 [XmlRoot(ElementName = nameof(Drug), IsNullable = false)]
-public class Drug
+public class Drug : NotifyBase
 {
 	#region fields
-	private int _quantity;
+	private int quantity;
 	#endregion
 
 	#region ctor
 	/// <summary>
-	/// Empty constructor for <see cref="Drug"/>
-	/// </summary>
-	public Drug()
-	{
-		Name = string.Empty;
-		Description = string.Empty;
-		CurrentPrice = default;
-		AveragePrice = default;
-		PurchasePrice = default;
-		Quantity = default;
-	}
-
-	/// <summary>
-	/// The <see cref="Enums.DrugType"/> constructor for <see cref="Drug"/>
-	/// </summary>
-	/// <param name="drugType"></param>
-	/// <param name="description"></param>
-	/// <param name="marketValue"></param>
-	public Drug(Enums.DrugType drugType, string description, int marketValue)
-	{
-		Name = drugType.GetName();
-		Description = description;
-		CurrentPrice = default;
-		AveragePrice = marketValue;
-		PurchasePrice = default;
-		Quantity = default;
-	}
-
-	/// <summary>
-	/// The <see cref="string"/> constructor for <see cref="Drug"/>
+	/// Initializes a new instance of the <see cref="Drug"/> class.
 	/// </summary>
 	/// <param name="drugName"></param>
 	/// <param name="description"></param>
@@ -56,54 +29,65 @@ public Drug(string drugName, string description, int marketValue)
 		AveragePrice = marketValue;
 		PurchasePrice = default;
 		Quantity = default;
+		PropertyChanged += OnDrugPropertyChanged;
 	}
 	#endregion
 
 	#region properties
 	/// <summary>
-	/// The <see cref="Name"/> property
+	/// The <see cref="Name"/> property is the name of the drug.
 	/// </summary>
 	[XmlText]
 	public string Name { get; set; }
 
 	/// <summary>
-	/// The <see cref="Description"/> property
+	/// The <see cref="Description"/> property is the description of the drug.
 	/// </summary>
 	[XmlIgnore]
 	public string Description { get; set; }
 
 	/// <summary>
-	/// The <see cref="CurrentPrice"/> property
+	/// The <see cref="CurrentPrice"/> property is the current price of the drug.
 	/// </summary>
 	[XmlIgnore]
 	public int CurrentPrice { get; set; }
 
 	/// <summary>
-	/// The <see cref="AveragePrice"/> property
+	/// The <see cref="AveragePrice"/> property is the normal market price of the drug.
 	/// </summary>
 	[XmlIgnore]
 	public int AveragePrice { get; set; }
 
 	/// <summary>
-	/// The <see cref="PurchasePrice"/> property
+	/// The <see cref="PurchasePrice"/> property is the price the drug has been purchased.
 	/// </summary>
 	[XmlAttribute(AttributeName = nameof(PurchasePrice))]
 	public int PurchasePrice { get; set; }
 
 	/// <summary>
-	/// The <see cref="Quantity"/> property, if the quantity gets set to 0 the <see cref="PurchasePrice"/> property goes 0 too
+	/// The <see cref="Quantity"/> property is the amount of the drug.
 	/// </summary>
 	[XmlAttribute(AttributeName = nameof(Quantity))]
-	public int Quantity
+	public int Quantity { get => quantity; set => SetProperty(ref quantity, value); }
+	#endregion
+
+	#region private methods
+	/// <summary>
+	/// The event trigger method if a property that notifies has changed.
+	/// </summary>
+	/// <param name="sender"></param>
+	/// <param name="e"></param>
+	private void OnDrugPropertyChanged(object sender, PropertyChangedEventArgs e)
 	{
-		get => _quantity;
-		set
+		if (sender is not Drug drug)
+			return;
+		if (e is null)
+			return;
+
+		if (Equals(e.PropertyName, nameof(drug.Quantity)))
 		{
-			_quantity = value;
-			if (value.Equals(0))
-			{
-				PurchasePrice = value;
-			}
+			if (Equals(drug.Quantity, 0))
+				drug.PurchasePrice = Quantity;
 		}
 	}
 	#endregion

Classes/PlayerStash.cs

@@ -5,58 +5,55 @@
 namespace Los.Santos.Dope.Wars.Classes;
 
 /// <summary>
-/// The <see cref="DealerStash"/> class for player types, inherits from <see cref="Stash"/>
+/// The <see cref="PlayerStash"/> class.
 /// </summary>
+/// <remarks>
+/// Inherits from the <see cref="Stash"/> base class.
+/// Implements the members of the <see cref="IPlayerStash"/> interface.
+/// </remarks>
 public class PlayerStash : Stash, IPlayerStash
 {
-	#region ctor
 	/// <summary>
-	/// The standard constructor for the <see cref="PlayerStash"/> class
+	/// Initializes a new instance of the <see cref="PlayerStash"/> class.
 	/// </summary>
-	public PlayerStash()
-	{
-		Drugs = new();
-	}
-	#endregion
+	public PlayerStash() => Drugs = new();
 
 	#region IPlayerStash members
-	/// <inheritdoc/>
+	/// <inheritdoc cref="IPlayerStash.BuyDrug(string, int, int)"/>
 	public void BuyDrug(string drugName, int drugQuantity, int drugPrice)
 	{
-		Drug drug = Drugs.Where(x => x.Name.Equals(drugName, StringComparison.Ordinal)).SingleOrDefault();
-
-		if (drug.Quantity.Equals(0))
-			drug.PurchasePrice = drugPrice;
-		else
-			drug.PurchasePrice = ((drug.Quantity * drug.PurchasePrice) + (drugQuantity * drugPrice)) / (drug.Quantity + drugQuantity);
-
+		CalculateNewPurchasePrice(drugName, drugQuantity, drugPrice);
 		AddToStash(drugName, drugQuantity);
-
 		Game.Player.Money -= drugPrice * drugQuantity;
 	}
-	/// <inheritdoc/>
+
+	/// <inheritdoc cref="IPlayerStash.MoveIntoInventory(string, int, int)"/>
 	public void MoveIntoInventory(string drugName, int drugQuantity, int drugPrice)
 	{
-		Drug drug = Drugs.Where(x => x.Name.Equals(drugName, StringComparison.Ordinal)).SingleOrDefault();
-
-		if (drug.Quantity.Equals(0))
-			drug.PurchasePrice = drugPrice;
-		else
-			drug.PurchasePrice = ((drug.Quantity * drug.PurchasePrice) + (drugQuantity * drugPrice)) / (drug.Quantity + drugQuantity);
-
+		CalculateNewPurchasePrice(drugName, drugQuantity, drugPrice);
 		AddToStash(drugName, drugQuantity);
 	}
-	/// <inheritdoc/>
+	/// <inheritdoc cref="IPlayerStash.SellDrug(string, int, int)"/>
 	public void SellDrug(string drugName, int drugQuantity, int drugPrice)
 	{
 		RemoveFromStash(drugName, drugQuantity);
-
 		Game.Player.Money += drugPrice * drugQuantity;
 	}
-	/// <inheritdoc/>
-	public void TakeFromInventory(string drugName, int drugQuantity)
-	{
+	/// <inheritdoc cref="IPlayerStash.TakeFromInventory(string, int)"/>
+	public void TakeFromInventory(string drugName, int drugQuantity) =>
 		RemoveFromStash(drugName, drugQuantity);
-	}
 	#endregion
+
+	/// <summary>
+	/// The method calculaties the new purchase price, when adding drug into the player stash. 
+	/// </summary>
+	/// <param name="drugName">The name of the drug.</param>
+	/// <param name="drugQuantity">The quantity / amount of the drug.</param>
+	/// <param name="drugPrice">The current price of the drug.</param>
+	private void CalculateNewPurchasePrice(string drugName, int drugQuantity, int drugPrice)
+	{
+		Drug drug = Drugs.Where(x => x.Name.Equals(drugName, StringComparison.Ordinal)).Single();
+		drug.PurchasePrice = drug.Quantity.Equals(0) ? drugPrice
+			: ((drug.Quantity * drug.PurchasePrice) + (drugQuantity * drugPrice)) / (drug.Quantity + drugQuantity);
+	}
 }

Interfaces/IPlayerStash.cs

@@ -1,38 +1,38 @@
 namespace Los.Santos.Dope.Wars.Interfaces;
 
 /// <summary>
-/// The <see cref="IPlayerStash"/> interface, provides methods specifically for the player
+/// The <see cref="IPlayerStash"/> interface, provides th epublic methods for the player stash.
 /// </summary>
 public interface IPlayerStash
 {
 	/// <summary>
-	/// The <see cref="BuyDrug(string, int, int)"/> method is used to manipulate the player stash
+	/// The method is used to manipulate the player stash by adding drugs through a buy transaction.
 	/// </summary>
-	/// <param name="drugName"></param>
-	/// <param name="drugQuantity"></param>
-	/// <param name="drugPrice"></param>
+	/// <param name="drugName">The name of the drug.</param>
+	/// <param name="drugQuantity">The quantity / amount of the drug.</param>
+	/// <param name="drugPrice">The current price of the drug.</param>
 	void BuyDrug(string drugName, int drugQuantity, int drugPrice);
 
 	/// <summary>
-	/// The <see cref="SellDrug(string, int, int)"/> method is used to manipulate the player stash
+	/// The method is used to manipulate the player stash by adding drugs through a sell transaction.
 	/// </summary>
-	/// <param name="drugName"></param>
-	/// <param name="drugQuantity"></param>
-	/// <param name="drugPrice"></param>
+	/// <param name="drugName">The name of the drug.</param>
+	/// <param name="drugQuantity">The quantity / amount of the drug.</param>
+	/// <param name="drugPrice">The current price of the drug.</param>
 	void SellDrug(string drugName, int drugQuantity, int drugPrice);
 
 	/// <summary>
-	/// The <see cref="MoveIntoInventory(string, int, int)"/> method is used to make transfers into player inventories
+	/// The method is used to make transfers into player inventories by adding drug into a inventory.
 	/// </summary>
-	/// <param name="drugName"></param>
-	/// <param name="drugQuantity"></param>
-	/// <param name="drugPrice"></param>
+	/// <param name="drugName">The name of the drug.</param>
+	/// <param name="drugQuantity">The quantity / amount of the drug.</param>
+	/// <param name="drugPrice">The current price of the drug.</param>
 	void MoveIntoInventory(string drugName, int drugQuantity, int drugPrice);
 
 	/// <summary>
-	/// The <see cref="TakeFromInventory(string, int)"/> method is used to make transfers from player inventories
+	/// The method is used to make transfers into player inventories by removing drug from a inventory.
 	/// </summary>
-	/// <param name="drugName"></param>
-	/// <param name="drugQuantity"></param>
+	/// <param name="drugName">The name of the drug.</param>
+	/// <param name="drugQuantity">The quantity / amount of the drug.</param>
 	void TakeFromInventory(string drugName, int drugQuantity);
 }