Implement read lock while accessing database.

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SQL-Server

ADO.NET

Oracle

Hi,
How we can achieve read lock in database access. I have a program with 4 instances of it, i want to implement read lock like:
-> when one instance access the table no other instance could not be accessed even for reading from the table.
-> Each instance can be used different transactions, only one transaction can have the access to the table. (only one transaction in one instance)
-> the table can be accessed only after committing the transaction used by the current instance.

I am doing this using ado.net. Now i am using
myTransaction = mySQLConnection->BeginTransaction(System::Data::IsolationLevel::Serializable);

this isolation level, but this will allow the other instances to read from the table, i wants to lock even the read. How can we achieve this.??? Is this possible in any way???