String comparison equivalents

Question

I believe these 2 lines are equivalent but after running into a strange issue I no longer believe this to be the case.

String mimeType = context.Request.ContentType;
(String.Compare("text/xml", mimeType, true) == 0))

is the same as :

context.Request.ContentType.ToLower().Equals("text/xml")

Are their implementations in the CLR any different?

Answer 1

They are not completely equivalent; see here.

Here is the correct way to do a case insensitive comparison:

bool areSame = str1.Equals(str2, StringComparison.OrdinalIgnoreCase);

This way will also be more efficient becasue it doesn't allocate a separate string for the lowercase copy.

Answer 2

They are not equivalent, and ToLower/ToUpper may have some localization issues. The way to compare two strings without case-sensitivity (considering one of the strings may be null, which is why I don't like the str1.Equals method) is the static String.Equals method:

bool areEqual = String.Equals(str1, str2, StringComparison.OrdinalIgnoreCase);

Answer 3

In addition to the other answers (@SLaks, @Serhio), I also feel obligated to point out that .ToLower() generates another string. Compare does not as far as I know. Excessive string generation in an app can come back to bite you in terms of memory usage and performance if it is in frequently called code.

Answer 4

the implementation of Compare(string, string, boolean) in .NET:

public static int Compare(string strA, string strB, bool ignoreCase)
{
    if (ignoreCase)
    {
        return CultureInfo.CurrentCulture.CompareInfo.Compare(strA, strB, CompareOptions.IgnoreCase);
    }
    return CultureInfo.CurrentCulture.CompareInfo.Compare(strA, strB, CompareOptions.None);
}

and Equals

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public bool Equals(string value)
{
    if ((value == null) && (this != null))
    {
        return false;
    }
    return EqualsHelper(this, value);
}

So, is NOT the same thing.