Imports System.Threading

Public Class ThreadSafe
    ' Field totalValue contains a running total that can be updated
    ' by multiple threads. It must be protected from unsynchronized 
    ' access.
    Private totalValue As Double = 0.0

    ' The Total property returns the running total.
    Public ReadOnly Property Total As Double
        Get
            Return totalValue
        End Get
    End Property

    ' AddToTotal safely adds a value to the running total.
    Public Function AddToTotal(ByVal addend As Double) As Double
        Dim initialValue, computedValue As Double
        Do
            ' Save the current running total in a local variable.
            initialValue = totalValue

            ' Add the new value to the running total.
            computedValue = initialValue + addend

            ' CompareExchange compares totalValue to initialValue. If
            ' they are not equal, then another thread has updated the
            ' running total since this loop started. CompareExchange
            ' does not update totalValue. CompareExchange returns the
            ' contents of totalValue, which do not equal initialValue,
            ' so the loop executes again.
        Loop While initialValue <> Interlocked.CompareExchange( _
            totalValue, computedValue, initialValue)
        ' If no other thread updated the running total, then 
        ' totalValue and initialValue are equal when CompareExchange
        ' compares them, and computedValue is stored in totalValue.
        ' CompareExchange returns the value that was in totalValue
        ' before the update, which is equal to initialValue, so the 
        ' loop ends.

        ' The function returns computedValue, not totalValue, because
        ' totalValue could be changed by another thread between
        ' the time the loop ends and the function returns.
        Return computedValue
    End Function
End Class

using System.Threading;

public class ThreadSafe {
    // totalValue contains a running total that can be updated
    // by multiple threads. It must be protected from unsynchronized 
    // access.
    private double totalValue = 0;

    // The Total property returns the running total.
    public double Total {
        get { return totalValue; }
    }

    // AddToTotal safely adds a value to the running total.
    public double AddToTotal(double addend) {
        double initialValue, computedValue;
        do {
            // Save the current running total in a local variable.
            initialValue = totalValue;

            // Add the new value to the running total.
            computedValue = initialValue + addend;

            // CompareExchange compares totalValue to initialValue. If
            // they are not equal, then another thread has updated the
            // running total since this loop started. CompareExchange
            // does not update totalValue. CompareExchange returns the
            // contents of totalValue, which do not equal initialValue,
            // so the loop executes again.
        } while (initialValue != Interlocked.CompareExchange(
            ref totalValue, computedValue, initialValue));
        // If no other thread updated the running total, then 
        // totalValue and initialValue are equal when CompareExchange
        // compares them, and computedValue is stored in totalValue.
        // CompareExchange returns the value that was in totalValue
        // before the update, which is equal to initialValue, so the 
        // loop ends.

        // The function returns computedValue, not totalValue, because
        // totalValue could be changed by another thread between
        // the time the loop ends and the function returns.
        return computedValue;
    }
}

Public Class ClassB
    ' The private field that stores the value for the
    ' ClassA property is intialized to Nothing. It is set
    ' once, from any of several threads. The field must
    ' be of type Object, so that CompareExchange can be
    ' used to assign the value. If the field is used
    ' within the body of class Test, it must be cast to
    ' type ClassA.
    Private classAValue As [Object] = Nothing
    ' This property can be set once to an instance of 
    ' ClassA. Attempts to set it again cause an
    ' exception to be thrown.
    
    Public Property ClassA() As ClassA
        Set
            ' CompareExchange compares the value in classAValue
            ' to Nothing. The new value assigned to the ClassA
            ' property, which is in the special variable 'value',
            ' is placed in classAValue only if classAValue is
            ' equal to Nothing.
            If Not (Nothing Is Interlocked.CompareExchange(classAValue, _
                    CType(value, [Object]), Nothing)) Then
                ' CompareExchange returns the original value of 
                ' classAValue; if it was not Nothing, then a value 
                ' was already assigned, and CompareExchange did not
                ' replace the original value. Throw an exception to
                ' indicate that an error occurred.
                Throw New ApplicationException("ClassA was already set.")
            End If
        End Set
        Get
            Return CType(classAValue, ClassA)
        End Get
    End Property
End Class ' ClassB

public class ClassB {
    // The private field that stores the value for the
    // ClassA property is intialized to null. It is set
    // once, from any of several threads. The field must
    // be of type Object, so that CompareExchange can be
    // used to assign the value. If the field is used
    // within the body of class Test, it must be cast to
    // type ClassA.
    private Object classAValue = null;
    // This property can be set once to an instance of 
    // ClassA. Attempts to set it again cause an
    // exception to be thrown.
    public ClassA ClassA {
        set {
            // CompareExchange compares the value in classAValue
            // to null. The new value assigned to the ClassA
            // property, which is in the special variable 'value',
            // is placed in classAValue only if classAValue is
            // equal to null.
            if (null != Interlocked.CompareExchange(ref classAValue,
                (Object) value, null)) {
                // CompareExchange returns the original value of 
                // classAValue; if it is not null, then a value 
                // was already assigned, and CompareExchange did not
                // replace the original value. Throw an exception to
                // indicate that an error occurred.
                throw new ApplicationException("ClassA was already set.");
            }
        }
        get {
            return (ClassA) classAValue;
        }
    }
}