Is there a case insensitive string replace in .Net without using Regex?

Question

I recently had to perform some string replacements in .net and found myself developing a regular expression replacement function for this purpose. After getting it to work I

Answer 1

This is a VB.NET adaptation of rboarman's answer above with the necessary checks for null and empty strings to avoid an infinite loop.

Public Function Replace(ByVal originalString As String, 
                        ByVal oldValue As String, 
                        ByVal newValue As String, 
                        ByVal comparisonType As StringComparison) As String
    If Not String.IsNullOrEmpty(originalString) AndAlso 
       Not String.IsNullOrEmpty(oldValue) AndAlso 
       newValue IsNot Nothing Then
        Dim startIndex As Int32

        Do While True
            startIndex = originalString.IndexOf(oldValue, startIndex, comparisonType)
            If startIndex = -1 Then Exit Do
            originalString = originalString.Substring(0, startIndex) & newValue & 
                             originalString.Substring(startIndex + oldValue.Length)
            startIndex += newValue.Length
        Loop
    End If

    Return originalString
End Function

Answer 2

Update in .NET Core 2.0+ (August 2017)

This is natively available in .NET Core 2.0⁺ with String.Replace which has the following overloads

public string Replace (string oldValue, string newValue, StringComparison comparisonType);
public string Replace (string oldValue, string newValue, bool ignoreCase, System.Globalization.CultureInfo culture);

So you can use either like this:

"A".Replace("a", "b", StringComparison.CurrentCultureIgnoreCase);
"A".Replace("a", "b", true, CultureInfo.CurrentCulture);

PS: You can browse .NET Core source if you want to see how MS implemented it

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Legacy .NET Framework 4.8^- option for VB Projects

Visual Basic has an Option Compare setting which can be set to Binary or Text

Setting to Text will make all string comparisons across your project case insensitive by default.

So, as other answers have suggested, if you are pulling in the Microsoft.VisualBasic.dll, when calling Strings.Replace if you don't explicitly pass a CompareMethod the method will actually defer to the Compare option for your file or project using the [OptionCompare] Parameter Attribute

So either of the following will also work (top option only available in VB , but both rely on VisualBasic.dll)

Option Compare Text

Replace("A","a","b")

Replace("A","a","b", Compare := CompareMethod.Text)

Answer 3

You can use Microsoft.VisualBasic.Strings.Replace and pass in Microsoft.VisualBasic.CompareMethod.Text to do a case insensitive replace like this:

Dim myString As String = "One Two Three"
myString = Replace(myString, "two", "TWO", Compare:= CompareMethod.Text)