Well... double quote ends a C# string ...
So you need to escape it if you want to pass a double quote to SQL:
SqlCommand scmd = new SqlCommand("EXEC sp_MSforeachtable \"ALTER TABLE ? NOCHECK CONSTRAINT all\"", con);
Or
SqlCommand scmd = new SqlCommand(@"EXEC sp_MSforeachtable ""ALTER TABLE ? NOCHECK CONSTRAINT all""", con);
But you might find that this works better with SQL:
SqlCommand scmd = new SqlCommand("EXEC sp_MSforeachtable 'ALTER TABLE ? NOCHECK CONSTRAINT all'", con);