update function in visual basic 2012 OOP method

See more:

VB

i'm working on a coding for update function in visual studio 2012 but, there is an "invalidcastexception was unhandled" error.

this is the coding for update function in windows form:

Private Sub btnupdate_Click(sender As Object, e As EventArgs) Handles btnupdate.Click
        ClassSiswa.Nis = txt_nis.Text
        ClassSiswa.Nisn = txt_nisn.Text
        ClassSiswa.Jenis_Kelamin = cmb_kelaminsiswa.Text
        ClassSiswa.Kota_Lahir = txt_kotalahir.Text
        ClassSiswa.Tanggal_Lahir = DTP_siswa.Text
        ClassSiswa.Agama = cmb_agamasiswa.Text
        ClassSiswa.Berat_Badan = txt_beratsiswa.Text
        ClassSiswa.Tinggi_Badan = txt_tinggi_badan.Text
        ClassSiswa.EditData(ClassSiswa.opencon, txt_nis.Text)
        MessageBox.Show(" Data Telah Diupdate")
        ClassKoneksi.closecon()
        datagridview()

    End Sub

and this is the class for the function:

Public Class ClassSiswa
    Inherits ClassKoneksi

    Private Shared _Nis, _Nisn, _Berat_Badan, _Tinggi_Badan As Integer
    Private Shared _Nama_Siswa, _Jenis_Kelamin, _Kota_Lahir, _Agama As String
    Private Shared _Tanggal_Lahir As Date
    Public Shared Property Nis() As Integer
        Get
            Return _Nis
        End Get
        Set(ByVal value As Integer)
            _Nis = value
        End Set
    End Property
    Public Shared Property Nisn() As Integer
        Get
            Return _Nisn
        End Get
        Set(ByVal value As Integer)
            _Nisn = value
        End Set
    End Property
    Public Shared Property Berat_Badan() As Integer
        Get
            Return _Berat_Badan
        End Get
        Set(ByVal value As Integer)
            _Berat_Badan = value
        End Set
    End Property
    Public Shared Property Tinggi_Badan() As Integer
        Get
            Return _Tinggi_Badan
        End Get
        Set(ByVal value As Integer)
            _Tinggi_Badan = value
        End Set
    End Property
    Public Shared Property Nama_Siswa() As String
        Get
            Return _Nama_Siswa
        End Get
        Set(ByVal value As String)
            _Nama_Siswa = value
        End Set
    End Property
    Public Shared Property Jenis_Kelamin() As String
        Get
            Return _Jenis_Kelamin
        End Get
        Set(ByVal value As String)
            _Jenis_Kelamin = value
        End Set
    End Property
    Public Shared Property Kota_Lahir() As String
        Get
            Return _Kota_Lahir
        End Get
        Set(ByVal value As String)
            _Kota_Lahir = value
        End Set
    End Property
    Public Shared Property Tanggal_Lahir() As Date
        Get
            Return _Tanggal_Lahir
        End Get
        Set(ByVal value As Date)
            _Tanggal_Lahir = value
        End Set
    End Property
    Public Shared Property Agama() As String
        Get
            Return _Agama
        End Get
        Set(ByVal value As String)
            _Agama = value
        End Set
    End Property
    
    Public Shared Sub EditData(ByVal _cn As SqlClient.SqlConnection, ByVal Nis As Integer)
        Dim sql As New SqlClient.SqlCommand
        sql.Connection = _cn
        sql.CommandType = CommandType.Text = "update siswa set Nis ='" & Nisn & "',Nama_Siswa='" & Nama_Siswa & "',Jenis_Kelamin='" & Jenis_Kelamin & "',Kota_Lahir='" & Kota_Lahir & "',Tanggal_Lahir='" & Tanggal_Lahir & "',Agama='" & Agama & "',Berat_Badan='" & Berat_Badan & "',Tinggi_Badan='" & Tinggi_Badan & "'where Nis='" & Nis & "'"
        ClassSiswa.cmd.ExecuteNonQuery()
        sql.ExecuteNonQuery()
    End Sub

and this is the SQL query:

Create Database KPIRWAN
use KPIRWAN

Create Table siswa 
(
Nis int,
Nisn int,
Nama_Siswa varchar(40),
Jenis_Kelamin varchar (10),
Kota_Lahir varchar (10),
Tanggal_Lahir date,
Agama varchar (10),
Berat_Badan int,
Tinggi_Badan int)

the function i want is after i searched the data with search button, the data can be changed or updated with update button.

the problem is:
in the btnupdate coding, when i tried to update data in form the error said
Conversion from string "" to type 'Integer' is not valid for:

ClassSiswa.Nis = txt_nis.Text

or

Conversion from string "" to type 'date' is not valid for:

ClassSiswa.Tanggal_Lahir = DTP_siswa.Text

how do fix this error?