Criar usuário e senha para acessar pelo Visual Studio / C#

Criei um banco, mas não consigo fazer conexão com ele pelo C#, não sei dizer se é algum problema com o usuário/senha que estou utilizando ou algo do gênero...

botão para testar:

using System.Data.SqlClient;

        private void buttonSqlServer_Click(object sender, EventArgs e)
        {
            string connectionString = null;
            SqlConnection connection;
            SqlCommand command;
            string sql = null;

            string server = textServidor.Text;
            string banco = textBanco.Text;
            string usuario = textUsuario.Text;
            string senha = textSenha.Text;

            connectionString = string.Format("Data Source={0};Initial Catalog={1};User ID={2};Password={3}",server,banco,usuario,senha);
            MessageBox.Show(connectionString);
            connection = new SqlConnection(connectionString);

            try
            {
                connection.Open();

                sql = "select * from tabela";

                command = new SqlCommand(sql, connection);
                command.ExecuteNonQuery();
                command.Dispose();
                connection.Close();
                MessageBox.Show(" ExecuteNonQuery in SqlCommand executed !!");
            }
            catch (Exception ex)
            {
                MessageBox.Show("Can not open connection !\n" + ex.ToString());
            }
        }
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Data Source=PROG_SAMIR\SQLEXPRESS;Initial Catalog=controle_compras;User ID=teste_user;Password=123456
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OK   
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Exception gerado:

---------------------------

---------------------------
Can not open connection !
System.Data.SqlClient.SqlException (0x80131904): Falha de logon do usuário 'teste_user'.

   em System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, DbConnectionPool pool, String accessToken, Boolean applyTransientFaultHandling)

   em System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions)

   em System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnection owningObject, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions)

   em System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection)

   em System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection)

   em System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection)

   em System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection)

   em System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)

   em System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)

   em System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)

   em System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry)

   em System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)

   em System.Data.SqlClient.SqlConnection.Open()

   em TesteSQL.FormPrincipal.buttonSqlServer_Click(Object sender, EventArgs e) na C:\Users\Samir\documents\visual studio 2015\Projects\TesteSQL\TesteSQL\Form1.cs:linha 137

ClientConnectionId:e76ca719-af59-4823-86b4-713629ee744c

Error Number:18456,State:1,Class:14
---------------------------
OK   
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