calculate MD5 or SHA checksum of each row from table fields

I am trying to write C# script to calculate MD5 hash for each row for certain fields in the table.

You may use HASHBYTES() T-SQL function and pass the concatenated columns as a parameter to it. You may use CONCAT() function to concatenate selected columns.

pcelba provides the solution, but expand for people looking into this and to give caution/context, you can use CHECKSUM or BINARY_CHECKSUM to generate a checksum for a row which I have seen in some databases.  While easier to work with, this is not the correct approach and your approach with generating a hash is the correct way (I would go at least SHA1 to avoid collisions).  

Using CHECKSUM or BINARY CHECKSUM you will have collisions, lots of them.  For example:

SELECT CHECKSUM(1.23),CHECKSUM(-1.23)

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both have a result of 2001159322.  So don't use these for any data version type requirement.