8.9 Dynamic Programming

Let’s go through dynamic typing / dynamic programming in C# — a feature that allows runtime type flexibility.

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📌 1. What is dynamic in C#?

• The dynamic keyword allows variables to bypass compile-time type checking.

• Type is resolved at runtime, rather than at compile-time like normal types (int, string, etc.).

• Useful when working with COM objects, reflection, JSON, or dynamic APIs.

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📌 2. Declaring Dynamic Variables

dynamic data = 10;
Console.WriteLine(data); // 10
 
data = "Hello";
Console.WriteLine(data); // Hello
 
data = DateTime.Now;
Console.WriteLine(data); // current date-time

• Variable type changes at runtime

• Compiler does not check method calls or property access

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📌 3. Dynamic vs Object

Feature	object	dynamic
Type Checking	Compile-time	Runtime
Method Calls	Need casting	No casting needed
Flexibility	Less flexible	Highly flexible
Errors	Compile-time errors	Runtime errors if invalid

Example:

object obj = 10;
// Console.WriteLine(obj + 5); // Compile-time error
 
dynamic dyn = 10;
Console.WriteLine(dyn + 5); // Works at runtime, output: 15

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📌 4. Using Dynamic in Methods

dynamic Add(dynamic a, dynamic b)
{
    return a + b;
}
 
Console.WriteLine(Add(5, 10));       // 15
Console.WriteLine(Add("Hello ", "C#")); // Hello C#

• Works with any types that support the operation at runtime

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📌 5. Dynamic and Reflection / COM

• dynamic is commonly used when types are unknown at compile time, e.g., working with:

  • COM objects (Excel, Word interop)

  • JSON or XML objects

  • Reflection-based method invocation

dynamic excelApp = Activator.CreateInstance(Type.GetTypeFromProgID("Excel.Application"));
excelApp.Visible = true;

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📌 6. Risks of dynamic

1. No compile-time checking → runtime errors

2. Performance overhead → runtime binding is slower

3. Should be used only when necessary, e.g., interacting with dynamic APIs

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📌 7. Summary Table

Feature	Description
Keyword	dynamic
Type Checking	Runtime
Flexibility	Can store any type
Common Uses	COM, reflection, JSON, XML, runtime API calls
Risks	Runtime errors, slower performance

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✅ Tip:

• Use dynamic for runtime-flexible scenarios

• Avoid dynamic when compile-time type safety is important