C# - Convert URL to plain text

When you use a string literal like that in C# the special characters are already processed in the code before any functions are called. So the unescape does nothing in that case. If you leave it out you get the same result:

MessageBox.Show("oJ\x2BkTRbYHjeME72JTd\x2F\x2Fnb\x2BKt0RWeAmk8Gzw7dOc\x2FAM\x3D:1438850410112:hEj6agVToFMpYj7MufF9Qg\x3D\x3D"); // ---> It "works".

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However, I suspect this is actually not the correct string. Look at it carefully below at *1*: there is a strange character that I don't "trust" (just before M=:).

How is the file generated? It does not look like it's generated with Uri.EscapeDataString() because then you would see %xx tokens in the string. In fact you can use the unescape if you replace "\x" with "%" in fileTxt:

MessageBox.Show(Uri.UnescapeDataString(fileTxt.Replace(@"\x", "%")));

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This gives a slightly different result (see *2* below) because of \x2FA which is processed as a double byte character in the literal string but probably needs to be processed as \x2F (a slash) followed by "AM" etc.

the 2 outputs:

*1* oJ+kTRbYHjeME72JTd//nb+Kt0RWeAmk8Gzw7dOc˺M=:1438850410112:hEj6agVToFMpYj7MufF9Qg==
*2* oJ+kTRbYHjeME72JTd//nb+Kt0RWeAmk8Gzw7dOc/AM=:1438850410112:hEj6agVToFMpYj7MufF9Qg==

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Be careful because it seems there's more going on: the file may contain 3 fields separated by colons because if it was 1 string I would expect they would have been encoded to \x3A. You may need to split it up first because if any of the fields contain an encoded colon then a split afterwards would result in more than 3 fields.