How can I convert ' character to " character in C#?

Quote:

For a sql string to be sent in C# I need to convert ' character inside the inside the string to " character.

No, you don't.

Based on the description, you're trying to "escape" special characters in a SQL query, because you're using string concatenation to add parameters to the query. But that leaves your code vulnerable to SQL Injection[^].

Instead, you need to use a parameterized query. That way, you don't need to worry about "special" characters, and you can just submit the values unchanged.

For example:

string theName = "Seamus O'Leary";

using (var connection = new SqlConnection("..."))
using (var command = new SqlCommand("SELECT SomeColumns FROM YourTable WHERE SomeName = @Name And SomeDate >= @MinDate", connection))
{
    command.Parameters.AddWithValue("@Name", theName);
    command.Parameters.AddWithValue("@MinDate", DateTime.Today);
    
    ...
}

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Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]