Comparative analysis of the difference between dynamic and var in C#

In the world of C# programming,varanddynamicThese two keywords often confuses beginners. They all seem to omit explicit type declarations when defining variables, but in fact they work in a very different way and application scenarios. Today, let’s uncover the mystery of these two keywords together.

var: Compilation-time type inference

varKeywords were introduced in C# 3.0 (2007), and its core feature isImplicit type, but that doesn't mean it's weak type. Instead, usevarThe declared variable is determined to be a specific strong type at compile time.

var name = "byteflow";   // The compiler infers name to string typevar age = 25;          // The compiler willageInferred asinttype

After compilation, the above code is equivalent to:

string name = "byteflow";
int age = 25;

Once the type is determined, it cannot be changed:

var number = 10;
// number = "ten";  // Compilation error！intThe type cannot be assigned asstring

dynamic: Type parsing at runtime

By comparison,dynamicIt was introduced in C# 4.0 (2010), and it represents a completely different programming paradigm. usedynamicThe variables declared by keywords willBypass compile-time type checking, all type checks are delayed toRuntimeconduct.

dynamic value = 100;
value = "Now I'm a string";  // Completely legal!value = new List<int>();  // No problem, either！

This means you can change the type of the variable at runtime or call methods that are not determined at compile time:

dynamic obj = GetSomeObject();  // We are not sure what type to return();              // Compiled by，Decide whether it can be called at runtime

Key Difference: Understand a Picture

When to use var?

varThe most suitable scenarios include:

Improve code readability, especially when dealing with long type names:

var dictionary = new Dictionary<string, List<Customer>>();
// Compare Dictionary<string, List<Customer>> dictionary = new Dictionary<string, List<Customer>>(); More concise

Handle anonymous types：

var person = new { Name = "byteflow", Age = 25 };

LINQ query results：

var results = from c in customers where  == "Beijing" select c;

When to use dynamic?

dynamicIt works in the following scenarios:

Interoperate with dynamic languages ​​or COM：

dynamic excel = ((""));
 = true;

Process JSON, XML and other data：

dynamic jsonData = (jsonString);
string name = ;  // No need to know in advanceJSONstructure

Replace complex reflections：

dynamic instance = (someType);
();  // Simpler than reflection code

Practical example: The same problem, two solutions

Suppose we need to process an object and print its properties:

How to use var(Safe at compile time):

void ProcessObject(Person person)
{
    var name = ;  // The compiler knows that this is a string type    var age = ;    // The compiler knows that this is the int type    ($"{name}This year{age}age");
}

How to use dynamic(More flexible but more risky):

void ProcessAnyObject(dynamic obj)
{
    try {
        var info = $"{}This year{}age";
        (info);
    }
    catch (RuntimeBinderException) {
        ("The object does not contain the required properties");
    }
}

Conclusion: Trade-offs and choices

varanddynamicRepresents two different design concepts of C# language: one is to ensure type safety but provide syntactic sugar to simplify code; the other is to provide dynamic features for enhanced flexibility.

As a rule of thumb: Used by defaultvarFor simplicity and performance, only used when dynamic behavior is really neededdynamic。

Remember, concise code is important, but type safety can often help you avoid many runtime errors that are difficult to debug. When choosing between the two, you need to weigh the gains and losses based on the specific scenario.

I hope this article can help you understand the essential differences between these two keywords and make smarter choices in C# programming!

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