Difference between String and string in .NET Core
💡 Concept Name
String vs string in C# / .NET Core
📘 Quick Intro
In C#, `string` is an alias (keyword) for `System.String`. Both represent the same .NET type. The choice between them is based on coding conventions or readability preferences.
🧠 Analogy / Short Story
Think of `string` and `String` like "bike" and "Bicycle". Both refer to the same object, just different naming styles—one's a casual word (alias), the other is formal (class name). Internally, they are identical.
🔧 Technical Explanation
In C#:
- `string` is a **C# keyword**, an alias for `System.String`.
- `String` refers to the **.NET class** `System.String` defined in the BCL (Base Class Library).
- They are functionally identical and interchangeable.
- Use `string` for variable declarations, and `String` when using static members like `String.IsNullOrEmpty()`.
🎯 Purpose & Use Case
- ✅ `string` - for declarations and fields
- ✅ `String` - for accessing static methods like `String.Format()`
- ✅ Both compile to `System.String`
- ✅ No performance difference
💻 Real Code Example
// Both lines declare the same type
string name = "Rishu";
String city = "Delhi";
// Both are valid and refer to same .NET type
Console.WriteLine(name.GetType()); // System.String
Console.WriteLine(String.IsNullOrEmpty(city)); // True or False
❓ Interview Q&A
Q1: Is `string` a class?
A: It is a keyword in C# that maps to the class `System.String`.
Q2: Are `string` and `String` interchangeable?
A: Yes, they are completely interchangeable in code.
Q3: Which is preferred for static methods?
A: Use `String` for calling static methods like `String.IsNullOrEmpty()`.
Q4: Which is preferred for declarations?
A: `string` is preferred for readability and consistency in declarations.
Q5: Are there performance differences?
A: No, they compile to the same IL code.
Q6: Can I mix usage of `string` and `String`?
A: Yes, but consistent style is better for readability.
Q7: Is `String` part of System namespace?
A: Yes, it’s `System.String`.
Q8: Is `string` available in VB.NET?
A: No, VB.NET uses `String` only.
Q9: Can I use `String s = null;`?
A: Yes, it’s valid syntax.
Q10: What’s a good convention?
A: Use `string` for type declarations, `String` for static method calls.
📝 MCQs
Q1. What does `string` represent in C#?
- A C++ class
- Alias for System.String
- Static class
- Value type
Q2. Which namespace does `String` belong to?
- CSharp
- DotNet
- System
- Core
Q3. Which is recommended for static methods?
- string
- String
- str
- None
Q4. Is `string` a data type?
- Yes, it’s a class
- No, it’s an interface
- It's an alias
- It's a struct
Q5. What will `typeof(string)` return?
- string
- System.String
- String
- string.class
Q6. Are `string` and `String` case-sensitive different types?
- Yes
- No
- Only in .NET Core
- Only in Framework
Q7. Which one is a .NET class?
- string
- String
- str
- StringType
Q8. Which is a C# keyword?
- System.String
- string
- System.string
- String()
Q9. Can you write `String s = null;`?
- No
- Yes
- Only in VB.NET
- Only with var
Q10. Do they have performance differences?
- Yes
- Sometimes
- No
- In .NET Core only
💡 Bonus Insight
Using `string` for declarations is a widely accepted C# convention, as it improves readability. Reserve `String` for static method access like `String.Concat`, `String.Format`, etc.
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