.NET无构造函数类字段默认值解析及Null赋值场景咨询

Let’s break this down clearly—since both of your questions tie together, I’ll wrap them into a straightforward, practical explanation.

When you create a C# class without defining any explicit constructors, the .NET runtime automatically generates a parameterless default constructor behind the scenes. This constructor’s sole job is to initialize every field in the class to its type-specific default value. Here’s exactly what that looks like for different types:

Value Types (structs, primitives like int/bool, etc.) #

All value types get set to their "zero equivalent":

• Numeric types (int, long, float, double, decimal): Default to 0 (or 0.0 for floating-point/decimal variants)
• bool: Defaults to false
• char: Defaults to '\0' (the null character)
• Custom struct types: Each field inside the struct is recursively initialized to its own default value (e.g., a Point struct with int X and int Y would have both X and Y set to 0)

Reference Types (classes, strings, arrays, delegates, etc.) #

Every reference type field defaults to null. This includes:

• Built-in reference types like string, List<T>, Dictionary<TKey, TValue>
• Your own custom class types
• Arrays and delegates

And to answer your direct question: Yes, fields absolutely can be set to null here. For example, a string field in your class will be null (not an empty string) unless you explicitly initialize it, and a custom class field will be null until you instantiate it separately.

Example Code to Visualize This #

public class UserProfile
{
    // Value type fields
    public int UserId;
    public bool HasPremiumAccess;
    public DateTime JoinDate; // DateTime is a struct, defaults to 01/01/0001 00:00:00

    // Reference type fields
    public string DisplayName;
    public List<string> SavedPosts;
}

// Instantiate without a custom constructor
var newUser = new UserProfile();

// Values after initialization:
// UserId = 0
// HasPremiumAccess = false
// JoinDate = 01/01/0001 00:00:00
// DisplayName = null
// SavedPosts = null

To recap: The auto-generated default constructor ensures no field is left uninitialized. Value types get their zero-value, reference types get null.

内容的提问来源于stack exchange，提问作者Leigha Brown