How to know if an HTTP request header value exists

by 
if (Request.Headers["XYZComponent"].Count() > 0)

… will attempted to count the number of characters in the returned string, but if the header doesn’t exist it will return NULL, hence why it’s throwing an exception. Your second example effectively does the same thing, it will search through the collection of Headers and return NULL if it doesn’t exist, which you then attempt to count the number of characters on:

Use this instead:

if(Request.Headers["XYZComponent"] != null)

Or if you want to treat blank or empty strings as not set then use:

if((Request.Headers["XYZComponent"] ?? "").Trim().Length > 0)

The Null Coalesce operator ?? will return a blank string if the header is null, stopping it throwing a NullReferenceException.

A variation of your second attempt will also work:

if (Request.Headers.AllKeys.Any(k => string.Equals(k, "XYZComponent")))

Edit: Sorry didn’t realise you were explicitly checking for the value true:

bool isSet = Boolean.TryParse(Request.Headers["XYZComponent"], out isSet) && isSet;

Will return false if Header value is false, or if Header has not been set or if Header is any other value other than true or false. Will return true is the Header value is the string ‘true’

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