Use the string.Compare method. Its third parameter specifies case sensitivity.

"fred" == "Fred" // false
string.Compare( "fred", "Fred", StringComparison.CurrentCultureIgnoreCase ) == 0 // true
For more control over the comparison, e.g. exotic features like width-sensitivity, consider using System.Globalization.CompareInfo.Compare(), e.g.

CultureInfo.CurrentCulture.CompareInfo.Compare(
"fred", "Fred",
CompareOptions.IgnoreCase |
CompareOptions.IgnoreKanaType |
CompareOptions.IgnoreWidth
);