Case insensitive 'Contains(string)'

You could use the String.IndexOf Method and pass StringComparison.OrdinalIgnoreCase as the type of search to use:

string title = "STRING";
bool contains = title.IndexOf("string", StringComparison.OrdinalIgnoreCase) >= 0;

Even better is defining a new extension method for string:

public static class StringExtensions
{
    public static bool Contains(this string source, string toCheck, StringComparison comp)
    {
        return source.IndexOf(toCheck, comp) >= 0;
    }
}

...

string title = "STRING";
bool contains = title.Contains("string", StringComparison.OrdinalIgnoreCase);

You can use IndexOf() like this:

string title = "STRING";

if (title.IndexOf("string", 0, StringComparison.CurrentCultureIgnoreCase) != -1)
{
    // The string exists in the original
}

Since 0 (zero) can be an index, you check against -1.

MSDN

The zero-based index position of value if that string is found, or -1 if it is not. If value is String.Empty, the return value is 0.

Alternate solution using Regex:

bool contains = Regex.IsMatch("StRiNG to search", "string", RegexOptions.IgnoreCase);

Notice

As @cHao has pointed out in his comment, there are scenario's that will cause this solution to return incorrect results. Make sure you know what you're doing before you implement this solution haphazardly.

One issue with the answer is that it will throw an exception if a string is null. You can add that as a check so it won't:

public static bool Contains(this string source, string toCheck, StringComparison comp)
{
    if (string.IsNullOrEmpty(toCheck) || string.IsNullOrEmpty(source))
        return true;

    return source.IndexOf(toCheck, comp) >= 0;
} 

You could always just up or downcase the strings first.

string title = "string":
title.ToUpper().Contains("STRING")  // returns true

Oops, just saw that last bit. A case insensitive compare would *probably* do the same anyway, and if performance is not an issue, I don't see a problem with creating uppercase copies and comparing those. I could have sworn that I once saw a case-insensitive compare once...

StringExtension class is the way forward, I've combined a couple of the posts above to give a complete code example:

public static class StringExtensions
{
    /// <summary>
    /// Allows case insensitive checks
    /// </summary>
    public static bool Contains(this string source, string toCheck, StringComparison comp)
    {
        return source.IndexOf(toCheck, comp) >= 0;
    }
}

This is clean and simple.

Regex.IsMatch(file,fileNamestr,RegexOptions.IgnoreCase)

OrdinalIgnoreCase, CurrentCultureIgnoreCase or InvariantCultureIgnoreCase?

Since this is missing, here are some recommendations about when to use which one:

Dos

• Use StringComparison.OrdinalIgnoreCase for comparisons as your safe default for culture-agnostic string matching.
• Use StringComparison.OrdinalIgnoreCase comparisons for increased speed.
• Use StringComparison.CurrentCulture-based string operations when displaying the output to the user.
• Switch current use of string operations based on the invariant culture to use the non-linguistic StringComparison.Ordinal or StringComparison.OrdinalIgnoreCase when the comparison is
  linguistically irrelevant (symbolic, for example).
• Use ToUpperInvariant rather than ToLowerInvariant when normalizing strings for comparison.

Don'ts

• Use overloads for string operations that don't explicitly or implicitly specify the string comparison mechanism.
• Use StringComparison.InvariantCulture -based string
  operations in most cases; one of the few exceptions would be
  persisting linguistically meaningful but culturally-agnostic data.

Based on these rules you should use:

string title = "STRING";
if (title.IndexOf("string", 0, StringComparison.[YourDecision]) != -1)
{
    // The string exists in the original
}

whereas [YourDecision] depends on the recommendations from above.

link of source: http://msdn.microsoft.com/en-us/library/ms973919.aspx

I know that this is not the C#, but in the framework (VB.NET) there is already such a function

Dim str As String = "UPPERlower"
Dim b As Boolean = InStr(str, "UpperLower")

C# variant:

string myString = "Hello World";
bool contains = Microsoft.VisualBasic.Strings.InStr(myString, "world");

The InStr method from the VisualBasic assembly is the best if you have a concern about internationalization (or you could reimplement it). Looking at in it dotNeetPeek shows that not only does it account for caps and lowercase, but also for kana type and full- vs. half-width characters (mostly relevant for Asian languages, although there are full-width versions of the Roman alphabet too). I'm skipping over some details, but check out the private method InternalInStrText:

private static int InternalInStrText(int lStartPos, string sSrc, string sFind)
{
  int num = sSrc == null ? 0 : sSrc.Length;
  if (lStartPos > num || num == 0)
    return -1;
  if (sFind == null || sFind.Length == 0)
    return lStartPos;
  else
    return Utils.GetCultureInfo().CompareInfo.IndexOf(sSrc, sFind, lStartPos, CompareOptions.IgnoreCase | CompareOptions.IgnoreKanaType | CompareOptions.IgnoreWidth);
}

Just like this:

string s="AbcdEf";
if(s.ToLower().Contains("def"))
{
    Console.WriteLine("yes");
}

Use this:

string.Compare("string", "STRING", new System.Globalization.CultureInfo("en-US"), System.Globalization.CompareOptions.IgnoreCase);

Ultimately, a generic "contains" operation comes down to a function like this,

/// <summary>
/// Determines whether the source contains the sequence.
/// </summary>
/// <typeparam name="T">The type of the items in the sequences.</typeparam>
/// <param name="sourceEnumerator">The source enumerator.</param>
/// <param name="sequenceEnumerator">The sequence enumerator.</param>
/// <param name="equalityComparer">An equality comparer.</param>
/// <remarks>
/// An empty sequence will return <c>true</c>.
/// The sequence must support <see cref="IEnumerator.Reset"/>
/// if it does not begin the source.
/// </remarks>
/// <returns>
/// <c>true</c> if the source contains the sequence;
/// otherwise <c>false</c>.
/// </returns>
public static bool Contains<T>(
    IEnumerator<T> sourceEnumerator,
    IEnumerator<T> sequenceEnumerator,
    IEqualityComparer<T> equalityComparer)
{
    if (equalityComparer == null)
    {
        equalityComparer = EqualityComparer<T>.Default;
    }

    while (sequenceEnumerator.MoveNext())
    {
        if (sourceEnumerator.MoveNext())
        {
            if (!equalityComparer.Equals(
                sourceEnumerator.Current,
                sequenceEnumerator.Current))
            {
                sequenceEnumerator.Reset();
            }
        }
        else
        {
            return false;
        }
    }

    return true;
}

this can be trivially wrapped in a extension version accepting IEnumerable like this,

public static bool Contains<T>(
        this IEnumerable<T> source,
        IEnumerable<T> sequence,
        IEqualityComparer<T> equalityComparer = null)
{
    if (sequence == null)
    {
        throw new ArgumentNullException("sequence");
    }

    using(var sequenceEnumerator = sequence.GetEnumerator())
    using(var sourceEnumerator = source.GetEnumerator())
    {
        return Contains(
            sourceEnumerator,
            sequenceEnumerator,
            equalityComparer);
    }
}

Now, this will work for the ordinal comparison of any sequences, including strings, since string implements IEnumerable<char>,

// The optional parameter ensures the generic overload is invoked
// not the string.Contains() implementation.
"testable".Contains("est", EqualityComparer<char>.Default)

However, as we know, strings are not generic, they are specialized. There are two key factors at play.

1. The "casing" issue which itself has various language dependent edge cases.
2. The rather involved issue of how a set of "Text Elements" (letters/numbers/symbols etc.) are represented by Unicode Code Points and what valid sequences of chars can represent a given string, details are expanded in these answers.

The net effect is the same. Strings that you might assert are linguistically equal can be validly represented by different combinations of chars. Whats more, the rules for validity change between cultures.

All this leads to a specialized string based "Contains" implementation like this.

using System.Globalization;

public static bool Contains(
         this string source,
         string value,
         CultureInfo culture = null,
         CompareOptions options = CompareOptions.None)
{
    if (value == null)
    {
        throw new ArgumentNullException("value");
    }

    var compareInfo = culture == null ? 
            CultureInfo.CurrentCulture.CompareInfo :
            culture.CompareInfo;

    var sourceEnumerator = StringInfo.GetTextElementEnumerator(source);
    var sequenceEnumerator = StringInfo.GetTextElementEnumerator(value);

    while (sequenceEnumerator.MoveNext())
    {
        if (sourceEnumerator.MoveNext())
        {
            if (!(compareInfo.Compare(
                    sourceEnumerator.Current,
                    sequenceEnumerator.Current,
                    options) == 0))
            {
                sequenceEnumerator.Reset();
            }
        }
        else
        {
            return false;
        }
    }

    return true;
}

This function can be used to perform a case insensitive, culture specific "contains" that will work, whatever the normalization of the strings. e.g.

"testable".Contains("EST", StringComparer.CurrentCultureIgnoreCase)

This is quite similar to other example here, but I've decided to simplify enum to bool, primary because other alternatives are normally not needed. Here is my example:

public static class StringExtensions
{
    public static bool Contains(this string source, string toCheck, bool bCaseInsensitive )
    {
        return source.IndexOf(toCheck, bCaseInsensitive ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal) >= 0;
    }
}

And usage is something like:

if( "main String substring".Contains("SUBSTRING", true) )
....

Using a RegEx is a straight way to do this:

Regex.IsMatch(title, "string", RegexOptions.IgnoreCase);

The trick here is to look for the string, ignoring case, but to keep it exactly the same (with the same case).

 var s="Factory Reset";
            var txt="reset";
 int first = s.IndexOf(txt, StringComparison.InvariantCultureIgnoreCase) + txt.Length;
 var subString = s.Substring(first - txt.Length, txt.Length);

Output is "Reset"

if ("strcmpstring1".IndexOf(Convert.ToString("strcmpstring2"), StringComparison.CurrentCultureIgnoreCase) >= 0){return true;}else{return false;}

You can use string.indexof () function. This will be case insensitive