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问题:

Can anybody tell me what is the default value of a variable at the time of declaration in C# and vb??

回答1:

In c# you can use the default keyword to determine default values.

For example:

default(bool)
default(int)
default(int?)

回答2:

Do you mean a (method) variable? or a field (on an instance or type)?

For a method-level variable (in C# at least) it is irrelevant, since "definite assignment" means that you must give it a value before you can read it.

Fields default to the bitwise zero state:

for reference types (including string) that means null
for Nullable<T> (int? etc) that means null
for numerics that means 0
for enums that means 0 even if there is no 0-valued enum defined
for bools that means false
for DateTime, that means the same as MinValue
for other structs, you'll have to check their documentation, but it will be a (hopefully sensible) "zero/empty" value

回答3:

This can be found in MSDN:

Visual Basic .NET defines the following primitive types:

The integral value types Byte (1-byte unsigned integer), Short (2-byte signed integer), Integer (4-byte signed integer), and Long (8-byte signed integer). These types map to System.Byte, System.Int16, System.Int32, and System.Int64, respectively. The default value of an integral type is equivalent to the literal 0.

The floating-point value types Single (4-byte floating point) and Double (8-byte floating point). These types map to System.Single and System.Double, respectively. The default value of a floating-point type is equivalent to the literal 0.

The Decimal type (16-byte decimal value), which maps to System.Decimal. The default value of decimal is equivalent to the literal 0D.

The Boolean value type, which represents a truth value, typically the result of a relational or logical operation. The literal is of type System.Boolean. The default value of the Boolean type is equivalent to the literal False.

The Date value type, which represents a date and/or a time and maps to System.DateTime. The default value of the Date type is equivalent to the literal # 01/01/0001 12:00:00AM #.

The Char value type, which represents a single Unicode character and maps to System.Char. The default value of the Char type is equivalent to the constant expression ChrW(0).

The String reference type, which represents a sequence of Unicode characters and maps to System.String. The default value of the String type is a null reference.

http://msdn.microsoft.com/en-us/library/aa711900.aspx

回答4:

The C# language specification states that for value types the default value is the same as the one assigned by the default constructor and for reference types it is null:

So the value types default constructor values are:

For all simple-types, the default value is the value produced by a bit pattern of all zeros:

For sbyte, byte, short, ushort, int, uint, long, and ulong, the default value is 0.

For char, the default value is '\x0000'. For float, the default
value is 0.0f.

For double, the default value is 0.0d. For decimal, the default value is 0.0m. For bool, the default value is false.

For an enum-typeE, the default value is 0.

For a struct-type, the default value is the value produced by setting all value type fields to their default value and all reference type fields to null.

http://msdn.microsoft.com/en-us/library/aa691142(v=VS.71).aspx

The string is not a value type.