C# is a statically-typed language, meaning that the types of variables are known at compile-time. However, in scenarios where the type of an object is not known until runtime, C# provides the dynamic keyword. This allows developers to defer type checking until runtime, offering more flexibility in situations like working with COM objects, dynamic languages, or when interacting with dynamic data sources like JSON or XML.
In this detailed guide, we will explore what dynamic
types are in C#, when and why to use them, and their potential pitfalls. We will also cover some real-world use cases with code examples, followed by key takeaways.
Dynamic
Type in C#?In C#, the dynamic keyword allows you to declare variables whose type is not determined until runtime. This contrasts with the var keyword, which infers the type at compile-time. When you use dynamic, the C# compiler skips type-checking for that variable at compile-time, and all the checks are deferred until the code runs.
Dynamic types provide a level of flexibility that isn’t possible with statically-typed variables. This can be particularly useful when you don’t know the exact type of an object in advance or are interacting with external components like COM objects, APIs, or dynamic languages like Python.
Dynamic
Variabledynamic myVar = "Hello, World!";
Console.WriteLine(myVar); // Outputs: Hello, World!
myVar = 100;
Console.WriteLine(myVar); // Outputs: 100
In this example, myVar
starts as a string, but later it is reassigned an int value. Since myVar
is dynamic
, C# allows this without type-checking at compile-time.
Dynamic
Types?While C# is strongly-typed, there are certain scenarios where the dynamic keyword is helpful:
dynamic
types make it easier to interact with these libraries without needing to define exact types.Dynamic
TypesDynamic
types are easy to declare and use. You simply replace the type of a variable with the dynamic
keyword, and C# will treat it as a dynamically-typed variable.
Dynamic
Variablesdynamic x = 10;
Console.WriteLine(x); // Outputs: 10
x = "Hello";
Console.WriteLine(x); // Outputs: Hello
x = DateTime.Now;
Console.WriteLine(x); // Outputs: Current Date and Time
Dynamic
TypesThe power of dynamic
types is that you can call any method or access any property without the compiler knowing the type. The validity of the call is checked at runtime, not at compile-time.
dynamic person = new { Name = "Alice", Age = 30 };
Console.WriteLine(person.Name); // Outputs: Alice
Console.WriteLine(person.Age); // Outputs: 30
Here, the compiler does not check whether Name
or Age
exist on person. This is validated at runtime.
Dynamic
Types vs Var
: What’s the Difference?The var
keyword is often confused with dynamic
, but they have very different behaviors.
var
is a compile-time feature. The type of a var
variable is inferred at compile-time, and after that, it becomes a statically-typed variable. You cannot change its type once inferred.dynamic
is a runtime feature. The type is resolved at runtime, meaning you can change the type or properties dynamically.var
vs dynamic
// Using var
var x = 10; // x is inferred to be an int
// x = "Hello"; // Compile-time error: Cannot assign string to int
// Using dynamic
dynamic y = 10;
y = "Hello"; // No error, type resolved at runtime
In this example, using var
enforces a strict type once it is inferred, whereas dynamic
allows the type to change at runtime.
One of the most common use cases of dynamic
is interacting with COM objects like Microsoft Office Interop, where the exact type of objects may not be known.
dynamic excelApp = Activator.CreateInstance(Type.GetTypeFromProgID("Excel.Application"));
excelApp.Visible = true;
dynamic workbook = excelApp.Workbooks.Add();
dynamic worksheet = workbook.Worksheets[1];
worksheet.Cells[1, 1] = "Hello, Excel!";
This example shows how dynamic
can simplify interactions with COM objects without needing to cast or use complex interop types.
When consuming APIs that return JSON, you might not always have a predefined class to deserialize into. In such cases, you can use dynamic
to work with the data easily.
string json = @"{ 'Name': 'John', 'Age': 30 }";
dynamic person = Newtonsoft.Json.JsonConvert.DeserializeObject(json);
Console.WriteLine(person.Name); // Outputs: John
Console.WriteLine(person.Age); // Outputs: 30
Here, the person
object is dynamically created based on the JSON structure without needing a predefined class.
When working with reflection, dynamic
types can simplify the process of invoking methods or accessing properties.
Type type = typeof(DateTime);
dynamic instance = Activator.CreateInstance(type);
Console.WriteLine(instance.ToString());
This code dynamically creates an instance of DateTime
and invokes its ToString()
method without needing to cast the object.
While dynamic
offers flexibility, it comes at a cost. The runtime must perform additional work to resolve method calls, property access, and other operations dynamically. This can introduce performance overhead, particularly in performance-critical applications or loops.
dynamic
, you lose compile-time type safety. Errors that could have been caught at compile-time (such as typos in method names) are only caught at runtime, potentially leading to runtime exceptions.Use dynamic only when necessary. For most use cases, statically-typed variables will provide better performance and safety.
When working with dynamic
types, Visual Studio will not offer IntelliSense suggestions for properties and methods. This can make it harder to discover members and catch mistakes.
Since type checking is deferred until runtime, you might encounter unexpected runtime errors if the object does not contain the members you expect.
dynamic obj = new { Name = "Alice" };
// Console.WriteLine(obj.Age); // Runtime error: 'Age' does not exist
You lose the benefits of compile-time type safety, making the code more error-prone and harder to maintain in larger projects.
As mentioned, the runtime overhead of dynamic resolution can impact performance in scenarios where dynamic
is used frequently or in tight loops.
var
is inferred at compile-time, dynamic
is resolved at runtime, allowing you to change types dynamically but at the cost of performance and safety.dynamic
sparingly, and prefer statically-typed code whenever possible to avoid runtime errors and performance penalties.C# dynamic
types are a powerful tool in situations where runtime flexibility is essential, such as when working with COM objects, dynamic languages, or JSON data. While they provide greater flexibility and reduce the need for complex type definitions, they come with the trade-off of losing compile-time type safety and potentially introducing performance overhead.
The key is to use dynamic types judiciously—embrace their power when needed, but avoid over-reliance in scenarios where compile-time safety and performance are more critical.
Understanding when and how to use dynamic types effectively can help you write cleaner, more adaptable code in situations that require runtime dynamism.