# C# 5.0 Syntax Requirements

You are working on a .NET Framework 4.8 project that MUST use C# 5.0 syntax only.

## Forbidden C# 6+ Features

### String Interpolation (C# 6)
❌ NEVER use: `$"Order {orderId} at {price}"`
✅ ALWAYS use: `string.Format("Order {0} at {1}", orderId, price)`

### Null-Conditional Operators (C# 6)
❌ NEVER use: `var name = order?.Name`
❌ NEVER use: `var value = dict?[key]`
✅ ALWAYS use explicit null checks:
```csharp
var name = order != null ? order.Name : null;
if (dict != null && dict.ContainsKey(key)) { }
```

### Null-Coalescing Assignment (C# 8)
❌ NEVER use: `value ??= defaultValue;`
✅ ALWAYS use: `if (value == null) value = defaultValue;`

### Expression-Bodied Members (C# 6)
❌ NEVER use: `public int Property => value;`
❌ NEVER use: `public void Method() => DoSomething();`
✅ ALWAYS use full syntax:
```csharp
public int Property 
{ 
    get { return value; } 
}

public void Method()
{
    DoSomething();
}
```

### nameof Operator (C# 6)
❌ NEVER use: `throw new ArgumentNullException(nameof(param));`
✅ ALWAYS use: `throw new ArgumentNullException("param");`

### Auto-Property Initializers (C# 6)
❌ NEVER use: `public int Property { get; set; } = 10;`
✅ ALWAYS use constructor initialization:
```csharp
public int Property { get; set; }

public ClassName()
{
    Property = 10;
}
```

### Using Static (C# 6)
❌ NEVER use: `using static System.Math;`
✅ ALWAYS use: `System.Math.Floor(...)`

### Tuple Syntax (C# 7)
❌ NEVER use: `var tuple = (name: "test", value: 1);`
✅ ALWAYS use: `Tuple<string, int>` or custom classes

### Pattern Matching (C# 7+)
❌ NEVER use: `if (obj is string str)`
✅ ALWAYS use: `if (obj is string) { var str = (string)obj; }`

### Local Functions (C# 7)
❌ NEVER use functions inside methods
✅ ALWAYS use private methods

### Out Variables (C# 7)
❌ NEVER use: `if (dict.TryGetValue(key, out var value))`
✅ ALWAYS use:
```csharp
OrderStatus value;
if (dict.TryGetValue(key, out value))
{
    // Use value
}
```

## Verification
After writing ANY code, verify C# 5.0 compliance:
- No `$` signs except in string literals
- No `?.` or `?[` operators
- No `=>` except in lambda expressions
- No inline variable declarations in out parameters