> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/SiveriusAlter/CurrencyExchange/llms.txt
> Use this file to discover all available pages before exploring further.

# Currencies

> Understanding currency models, validation rules, and ISO standards in the Currency Exchange API

## Overview

Currencies are fundamental entities in the Currency Exchange API. Each currency is represented by the `Currency` model, which enforces strict validation rules to ensure data integrity and compliance with international standards.

## Currency Model Structure

The `Currency` class uses an immutable design pattern with a private constructor and a public factory method for creation.

```csharp CurrencyExchange.Core/Models/Currency.cs theme={null}
public class Currency
{
    private Currency(int id, string code, string fullName, string sign)
    {
        Id = id;
        Code = code;
        FullName = fullName;
        Sign = sign;
    }

    public int Id { get; }
    public string Code { get; }
    public string FullName { get; }
    public string Sign { get; }
}
```

### Properties

<ResponseField name="Id" type="int" required>
  Unique identifier for the currency
</ResponseField>

<ResponseField name="Code" type="string" required>
  ISO 4217 currency code (e.g., "USD", "EUR", "RUB")
</ResponseField>

<ResponseField name="FullName" type="string" required>
  Full name of the currency (e.g., "US Dollar", "Euro")
</ResponseField>

<ResponseField name="Sign" type="string" required>
  Currency symbol (e.g., "\$", "€", "₽")
</ResponseField>

## Creating Currencies

Currencies are created using the static `Create` method, which performs all necessary validations:

```csharp theme={null}
public static Currency Create(int id, string code, string fullName, string sign)
{
    code = code.ToUpperInvariant();

    Validate(code, 3, 5, @"[^A-Z]");
    Validate(fullName, 3, 60, @"[^A-Za-zА-Яа-яЁё() ]");
    Validate(sign, 1, 3, @"[ \n]");

    return new Currency(id, code, fullName, sign);
}
```

<Note>
  The currency code is automatically converted to uppercase before validation, ensuring consistency across the system.
</Note>

## Validation Rules

The Currency model enforces strict validation rules using regular expressions and length constraints:

<Accordion title="Currency Code Validation">
  **Length:** 3-5 characters

  **Pattern:** Only uppercase letters (A-Z)

  **Regex:** `[^A-Z]` (rejects any non-uppercase letter)

  **Examples:**

  * ✅ Valid: "USD", "EUR", "RUB", "GBP"
  * ❌ Invalid: "us" (too short), "usd" (lowercase), "U\$D" (special character)

  ```csharp theme={null}
  Validate(code, 3, 5, @"[^A-Z]");
  ```
</Accordion>

<Accordion title="Full Name Validation">
  **Length:** 3-60 characters

  **Pattern:** Letters (Latin and Cyrillic), parentheses, and spaces

  **Regex:** `[^A-Za-zА-Яа-яЁё() ]` (rejects anything except allowed characters)

  **Examples:**

  * ✅ Valid: "US Dollar", "Euro", "Российский рубль"
  * ❌ Invalid: "\$" (too short), "Currency\@123" (special characters)

  ```csharp theme={null}
  Validate(fullName, 3, 60, @"[^A-Za-zА-Яа-яЁё() ]");
  ```
</Accordion>

<Accordion title="Currency Sign Validation">
  **Length:** 1-3 characters

  **Pattern:** No spaces or newlines

  **Regex:** `[ \n]` (rejects spaces and newlines)

  **Examples:**

  * ✅ Valid: "\$", "€", "₽", "£"
  * ❌ Invalid: "" (empty), "\$ " (contains space)

  ```csharp theme={null}
  Validate(sign, 1, 3, @"[ \n]");
  ```
</Accordion>

## Validation Implementation

The validation logic is centralized in a static method that checks for null/empty values, length constraints, and pattern matching:

```csharp CurrencyExchange.Core/Models/Currency.cs theme={null}
public static void Validate(string validateString, int minLength, int maxLength, string pattern)
{
    if (string.IsNullOrEmpty(validateString))
        throw new ArgumentException("Не заполнен один или несколько параметров\n");
    else if (validateString.Length < minLength || validateString.Length > maxLength)
        throw new ArgumentException(
            $"Не корректное количество символов указано для поля {validateString} валюты. Длина строки может быть от {minLength}-х до {maxLength} символов.\n");
    else if (Regex.IsMatch(validateString, pattern, RegexOptions.Compiled))
        throw new ArgumentException($"Не корректные символы в строке {validateString}.\n");
}
```

<Tip>
  The validation method uses `RegexOptions.Compiled` for improved performance when validating multiple currencies.
</Tip>

## ISO 4217 Currency Codes

The Currency Exchange API follows ISO 4217 standards for currency codes. Common examples include:

| Code | Currency      | Symbol |
| ---- | ------------- | ------ |
| USD  | US Dollar     | \$     |
| EUR  | Euro          | €      |
| RUB  | Russian Ruble | ₽      |
| GBP  | British Pound | £      |
| JPY  | Japanese Yen  | ¥      |
| CNY  | Chinese Yuan  | ¥      |

<Note>
  While ISO 4217 standard codes are typically 3 characters, the API allows up to 5 characters to accommodate custom or future currency codes.
</Note>

## Error Handling

When validation fails, the `Create` method throws an `ArgumentException` with descriptive error messages:

```csharp theme={null}
try
{
    var currency = Currency.Create(1, "us", "US Dollar", "$");
}
catch (ArgumentException ex)
{
    // Error: "Не корректное количество символов указано для поля us валюты..."
}
```

## Best Practices

<Card title="Always Use the Create Method" icon="check">
  Never attempt to instantiate `Currency` directly. Always use the static `Create` method to ensure all validation rules are applied.
</Card>

<Card title="Handle Validation Errors" icon="exclamation-triangle">
  Wrap currency creation in try-catch blocks to handle validation errors gracefully and provide user-friendly error messages.
</Card>

<Card title="Use Standard ISO Codes" icon="globe">
  Stick to official ISO 4217 currency codes whenever possible to ensure compatibility with external systems and services.
</Card>