Где находится реализация интерфейсов в NET - C#

[TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    public int IndexOf(char value)
    {
        return this.IndexOf(value, 0, this.Length);
    }
 
    public int IndexOf(string value)
    {
        return this.IndexOf(value, StringComparison.CurrentCulture);
    }
 
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    public int IndexOf(char value, int startIndex)
    {
        return this.IndexOf(value, startIndex, this.Length - startIndex);
    }
 
    public int IndexOf(string value, int startIndex)
    {
        return this.IndexOf(value, startIndex, StringComparison.CurrentCulture);
    }
 
    [SecuritySafeCritical, TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    public int IndexOf(string value, StringComparison comparisonType)
    {
        return this.IndexOf(value, 0, this.Length, comparisonType);
    }
 
    [MethodImpl(MethodImplOptions.InternalCall), SecuritySafeCritical]
    public extern int IndexOf(char value, int startIndex, int count);
    public int IndexOf(string value, int startIndex, int count)
    {
        if ((startIndex < 0) || (startIndex > this.Length))
        {
            throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
        }
        if ((count < 0) || (count > (this.Length - startIndex)))
        {
            throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
        }
        return this.IndexOf(value, startIndex, count, StringComparison.CurrentCulture);
    }
 
    [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
    public int IndexOf(string value, int startIndex, StringComparison comparisonType)
    {
        return this.IndexOf(value, startIndex, this.Length - startIndex, comparisonType);
    }
 
    public int IndexOf(string value, int startIndex, int count, StringComparison comparisonType)
    {
        if (value == null)
        {
            throw new ArgumentNullException("value");
        }
        if ((startIndex < 0) || (startIndex > this.Length))
        {
            throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
        }
        if ((count < 0) || (startIndex > (this.Length - count)))
        {
            throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
        }
        switch (comparisonType)
        {
            case StringComparison.CurrentCulture:
                return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.None);
 
            case StringComparison.CurrentCultureIgnoreCase:
                return CultureInfo.CurrentCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.IgnoreCase);
 
            case StringComparison.InvariantCulture:
                return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.None);
 
            case StringComparison.InvariantCultureIgnoreCase:
                return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.IgnoreCase);
 
            case StringComparison.Ordinal:
                return CultureInfo.InvariantCulture.CompareInfo.IndexOf(this, value, startIndex, count, CompareOptions.Ordinal);
 
            case StringComparison.OrdinalIgnoreCase:
                return TextInfo.IndexOfStringOrdinalIgnoreCase(this, value, startIndex, count);
        }
        throw new ArgumentException(Environment.GetResourceString("NotSupported_StringComparison"), "comparisonType");
    }