using System;

class Sample {
  static void Main()
  {
    int i = default(int); // intのデフォルト値
    bool b = default(bool); // boolのデフォルト値
    string s = default(string); // stringのデフォルト値
    DateTime dtm = default(DateTime); // DateTimeのデフォルト値
    Point p = default(Point); // 独自に定義した構造体Pointのデフォルト値

    Console.WriteLine("default(int) = {0}", i);
    Console.WriteLine("default(bool) = {0}", b);
    Console.WriteLine("default(string) = {0}, null? = {1}", s, s == null);
    Console.WriteLine("default(DateTime) = {0}", dtm);
    Console.WriteLine("default(Point) = {0}", p);
  }
}

struct Point {
  public Point(int x, int y)
  {
    this.x = x;
    this.y = y;
  }

  public override string ToString()
  {
    return string.Format("({0}, {1})", x, y);
  }

  private int x, y;
}

default(int) = 0
default(bool) = False
default(string) = , null? = True
default(DateTime) = 0001/01/01 0:00:00
default(Point) = (0, 0)

Imports System

Class Sample
  Shared Sub Main()
    Dim i As Integer = Nothing ' Integerのデフォルト値
    Dim b As Boolean = Nothing ' Booleanのデフォルト値
    Dim s As String = Nothing ' Stringのデフォルト値
    Dim dtm As Date = Nothing ' Dateのデフォルト値
    Dim p As Point = Nothing ' 独自に定義した構造体Pointのデフォルト値

    Console.WriteLine("default(Integer) = {0}", i)
    Console.WriteLine("default(Boolean) = {0}", b)
    Console.WriteLine("default(String) = {0}, Is Nothing? = {1}", s, s Is Nothing)
    Console.WriteLine("default(Date) = {0}", dtm)
    Console.WriteLine("default(Point) = {0}", p)
  End Sub
End Class

Structure Point
  Public Sub New(ByVal x As Integer, ByVal y As Integer)
    Me.x = x
    Me.y = y
  End Sub

  Public Overrides Function ToString() As String
    Return String.Format("({0}, {1})", x, y)
  End Function

  Private x, y As Integer
End Structure

default(Integer) = 0
default(Boolean) = False
default(String) = , Is Nothing? = True
default(Date) = 0001/01/01 0:00:00
default(Point) = (0, 0)

using System;

class Sample {
  // 任意の型Tの配列の内容をクリアする
  static void Clear<T>(T[] arr)
  {
    for (int i = 0; i < arr.Length; i++) {
      arr[i] = default(T); // 型Tのデフォルト値を設定する
      arr[i] = default;    // 対象の型が明らかな場合は型指定を省略することもできる
    }
  }

  static void Main()
  {
    var intArray = new int[10];
    var strArray = new string[10];

    Clear(intArray);
    Clear(strArray);
  }
}

Imports System

Class Sample
  ' 任意の型Tの配列の内容をクリアする
  Shared Sub Clear(Of T)(ByVal arr As T())
    For i As Integer = 0 To arr.Length - 1
      arr(i) = Nothing ' 型Tのデフォルト値を設定する
    Next
  End Sub

  Shared Sub Main()
    Dim intArray(9) As Integer
    Dim strArray(9) As String

    Clear(Of Integer)(intArray)
    Clear(Of String)(strArray)
  End Sub
End Class

using System;

class Sample {
  static void Main()
  {
    const double zero = 0.0;
    const double plus = +1.0;
    const double minus = -1.0;

    // ゼロによる除算・定数フィールドから無限大を得る
    Console.WriteLine("{0} {1}", plus / zero, double.PositiveInfinity);
    Console.WriteLine("{0} {1}", minus / zero, double.NegativeInfinity);

    // ゼロ同士の除算・定数フィールドから非数を得る
    Console.WriteLine("{0} {1}", zero / zero, double.NaN);
  }
}

Imports System

Class Sample
  Shared Sub Main()
    Const zero As Double = 0.0
    Const plus As Double = +1.0
    Const minus As Double = -1.0

    ' ゼロによる除算・定数フィールドから無限大を得る
    Console.WriteLine("{0} {1}", plus / zero, double.PositiveInfinity)
    Console.WriteLine("{0} {1}", minus / zero, double.NegativeInfinity)

    ' ゼロ同士の除算・定数フィールドから非数を得る
    Console.WriteLine("{0} {1}", zero / zero, double.NaN)
  End Sub
End Class

using System;

class Sample {
  static void IsSpecialValue(double d)
  {
    // 値が無限大かどうか、非数かどうかを調べる
    Console.WriteLine("[Value: {0}]", d);
    Console.WriteLine("IsInfinity? {0}", double.IsInfinity(d));
    Console.WriteLine("IsPositiveInfinity? {0}", double.IsPositiveInfinity(d));
    Console.WriteLine("IsNegativeInfinity? {0}", double.IsNegativeInfinity(d));
    Console.WriteLine("IsNaN? {0}", double.IsNaN(d));
    Console.WriteLine();
  }

  static void Main()
  {
    double d0 =  0.0;
    double d1 = +1.0 / 0.0;
    double d2 = -1.0 / 0.0;
    double d3 =  0.0 / 0.0;

    IsSpecialValue(d0);
    IsSpecialValue(d1);
    IsSpecialValue(d2);
    IsSpecialValue(d3);
  }
}

Imports System

Class Sample
  Shared Sub IsSpecialValue(ByVal d As Double)
    ' 値が無限大かどうか、非数かどうかを調べる
    Console.WriteLine("[Value: {0}]", d)
    Console.WriteLine("IsInfinity? {0}", Double.IsInfinity(d))
    Console.WriteLine("IsPositiveInfinity? {0}", Double.IsPositiveInfinity(d))
    Console.WriteLine("IsNegativeInfinity? {0}", Double.IsNegativeInfinity(d))
    Console.WriteLine("IsNaN? {0}", Double.IsNaN(d))
    Console.WriteLine()
  End Sub

  Shared Sub Main()
    Dim d0 As Double = 0.0
    Dim d1 As Double = +1.0 / 0.0
    Dim d2 As Double = -1.0 / 0.0
    Dim d3 As Double =  0.0 / 0.0

    IsSpecialValue(d0)
    IsSpecialValue(d1)
    IsSpecialValue(d2)
    IsSpecialValue(d3)
  End Sub
End Class

using System;

class Sample {
  static void Main()
  {
    const decimal zero = 0.0m;
    const decimal nonzero = 1.0m;
    decimal one = 1.0m;

    // 定数0による除算の場合、コンパイル時エラーとなる
    Console.WriteLine(nonzero / zero); // error CS0020: 定数 0 による除算です。

    // 実行時に0除算となる場合、例外DivideByZeroExceptionがスローされる
    Console.WriteLine(one / (one - 1.0m));
  }
}

Imports System

Class Sample
  Shared Sub Main()
    Const zero As Decimal = 0.0D
    Const nonzero As Decimal = 1.0D
    Dim one As Decimal = 1.0D

    ' 定数0による除算の場合、コンパイル時エラーとなる
    Console.WriteLine(nonzero / zero) ' error BC30542: この式の評価中に 0 による除算が発生しました。

    ' 実行時に0除算となる場合、例外DivideByZeroExceptionがスローされる
    Console.WriteLine(one / (one - 1.0D))
  End Sub
End Class