Create 521954 - BCL CultureAwareComparer with ignore casing on serial… (

microsoft#553)

* Create 521954 - BCL CultureAwareComparer with ignore casing on serialized on previous versions of .NET do not correctly deserialize on .NET 4.7.1.md

* Update 521954 - BCL CultureAwareComparer with ignore casing on serialized on previous versions of .NET do not correctly deserialize on .NET 4.7.1.md

* Update 521954 - BCL CultureAwareComparer with ignore casing on serialized on previous versions of .NET do not correctly deserialize on .NET 4.7.1.md

releases/net471/KnownIssues/521954 - BCL CultureAwareComparer with ignore casing on serialized on previous versions of .NET do not correctly deserialize on .NET 4.7.1.md

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+# CultureAwareComparer with ignore casing serialized on previous versions of .NET Framework does not correctly deserialize on .NET Framework 4.7.1 
+
+## Symptoms
+
+When deserialzing a CultureAwareComparer that was serialized using ignore case on a previous version of .NET Framework on a machine running .NET Framework 4.7.1, the comparer becomes case sensitive.
+
+## Cause
+
+In the .NET Framework 4.7.1 we added the ability to specify more comparer options when doing a culture-aware comparison. 
+As part of that change we introduced a new field on the comparer that holds the information about what kind of comparison is requested.
+
+When that type, coming from a version of .NET Framework prior to 4.7.1 is deserialized on a .NET Framework 4.7.1 machine, the private field introduced in the .NET Framework 4.7.1 is reset to zero, which makes the comparer case sensitive.
+
+## Impact
+
+The following 2 comparers are susceptible to this issue. These comparers are usually used with collection types like Dictionary and Hashtable.
+P:System.StringComparer.InvariantCultureIgnoreCase
+P:System.StringComparer.CurrentCultureIgnoreCase
+
+## Workarounds
+
+Ensure that both serialization and deserialization occurs on systems running a version of the .NET Framework starting with 4.7.1.