Visual Basic

<compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
  <application>
    <!-- A list of the Windows versions that this application has been tested on and is
         is designed to work with. Uncomment the appropriate elements and Windows will
         automatically selected the most compatible environment. -->

    <!-- Windows Vista -->
    <supportedOS Id="{e2011457-1546-43c5-a5fe-008deee3d3f0}" />

    <!-- Windows 7 -->
    <supportedOS Id="{35138b9a-5d96-4fbd-8e2d-a2440225f93a}" />

    <!-- Windows 8 -->
    <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}" />

    <!-- Windows 8.1 -->
    <supportedOS Id="{1f676c76-80e1-4239-95bb-83d0f6d0da78}" />

    <!-- Windows 10 -->
    <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}" />

  </application>
</compatibility>

Module Module1

    Sub Main()
        Dim iCount As Integer
        Dim strToSearch As String = "Every instance. Replace() will inteiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiirnally loop throughiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii the entire source string. It replaces all occurrences of the substring you want to replace. Tip: This behavior is useful when dealing with common string replacements in programs.  But: If you do not understand this behavior you may end up writing inefficient code that has unneeded loops. "
        Dim strSearchFor As String = "i"
        Console.WriteLine(strToSearch)
        iCount = strToSearch.Length() - strToSearch.Replace(strSearchFor, "").Length
        Console.WriteLine("There are " + iCount.ToString() + " letter " + strSearchFor + "'s in the string")

    End Sub

End Module

public class StringCharCountBenchmark
{
    private static readonly string _theStringToSearch = "Every instance. Replace() will inteiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiirnally loop throughiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii the entire source string. It replaces all occurrences of the substring you want to replace. Tip: This behavior is useful when dealing with common string replacements in programs.  But: If you do not understand this behavior you may end up writing inefficient code that has unneeded loops. ";
    private static readonly char _theCharToFind = 'i';
    private static readonly string _theCharToFindAsString = "i";

    [Benchmark]
    public int UsingReplace()
    {
        return _theStringToSearch.Length - _theStringToSearch.Replace(_theCharToFindAsString, "").Length;
    }

    [Benchmark]
    public int UsingForEachLoop()
    {
        int result = 0;
        foreach (char c in _theStringToSearch)
        {
            if (c == _theCharToFind)
            {
                result++;
            }
        }

        return result;
    }
    
    [Benchmark]
    public int UsingForLoop()
    {
        int result = 0;
        for (int i = 0; i < _theStringToSearch.Length; i++)
        {
            if (_theStringToSearch[i] == _theCharToFind)
            {
                result++;
            }
        }

        return result;
    }
}

(Yes, I know: C#. But I'm sure you can convert to VB.NET if required. )

// * Summary *

BenchmarkDotNet=v0.11.1, OS=Windows 10.0.17134.228 (1803/April2018Update/Redstone4)
Intel Core i7-4770K CPU 3.50GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
Frequency=3417969 Hz, Resolution=292.5714 ns, Timer=TSC
  [Host]     : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0
  DefaultJob : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3132.0


           Method |       Mean |     Error |    StdDev |
----------------- |-----------:|----------:|----------:|
     UsingReplace | 3,923.6 ns | 15.267 ns | 12.749 ns |
 UsingForEachLoop |   350.1 ns |  4.453 ns |  3.948 ns |
     UsingForLoop |   376.1 ns |  7.239 ns |  8.046 ns |

Module Module1

    Sub Main()

        Dim strToSearch As String = "john????"
        Dim strSearchFor As String = "?"
        Console.WriteLine(strToSearch)

        Console.WriteLine("There are " + CountOccurrences(strToSearch, strSearchFor).ToString() + " occurrences of " + strSearchFor + " in the string")

    End Sub

    Function CountOccurrences(strStringToSearch As String, strStringToCount As String) As Integer
        Dim iCount As Integer
        iCount = strStringToSearch.Length() - strStringToSearch.Replace(strStringToCount, "").Length
        Return iCount
    End Function

End Module