If a column is configured to receive text, that is, if its data type is set to string, by default, it can hold 0 to 32767 characters. This is (too) long for most cases. For example, if a column is made for people's names, this length is certainly too high. Fortunately, to customize the behavior of a column, you can limit the number of the characters that can be entered in a column.

To support the ability to control the number of characters that a text-based column would allow, the DataColumn class is equipped with a property named MaxLength. The default value of this property is -1, which means there is no limit.

To visually set the maximum length, in the Columns Collection Editor, click a column in the Members list. In the Properties list, click MaxLength and type the desired value. To programmatically specify the maximum length, assign an integer value to the MaxLength property of the column's variable.

When performing data entry, if the user does not have a value for a certain column, he or she may skip it. In some cases, you may want a value to be required; that is, you would not let the column to be left empty. When a column is left empty, it is referred to as null.

To support the ability to have a null value or to require it, the DataColumn class is equipped with a Boolean property named AllowDBNull. If you want the user to be able to skip a column and not provide a value, you can ignore this property or set it to true. To require a value for a column, set this property to false.

1. In the Members list, click DateAllocated
2. In the Properties list, double-click the value of the AllowDBNull field to set it to False
3. In the Members list, click LoanNumber
4. In the Properties list, double-click the value of the AllowDBNull field to set it to False
5. Click Close and click Close
6. To create a new form, in the Solution Explorer, right-click WattsALoan4 -> Add -> Windows Forms...
7. Set the Name to Employees and click Add
8. From the Data section of the Toolbox, click DataSet and click the form
9. Select the Untyped Dataset radio button and click OK
10. In the Properties window, change the following characteristics:
    DataSetName: dsEmployees
    (Name): Employees
11. Click Tables and click its ellipsis button
12. To create a new table, click Add and change the properties as follows:
    TableName: Employee
    (Name): tblEmployee
13. Click Columns and click its ellipsis button
14. Click Add 5 times and change the properties as follows:
     

    AllowDBNull	ColumnName	DefaultValue	DataType	Expression	Unique	(Name)
    False	EmployeeNumber				True	colEmployeeNumber
    	FirstName					colFirstName
    False	LastName					colLastName
    	Title					colTitle
    	HourlySalary	8.75	System.Double			colHourlySalary
    	EmployeeDetails			EmployeeNumber + ': ' +FirstName + ' ' + LastName		colEmployeeDetails

15. Click Close and click Close
16. Design the form as follows:
     

    Control Text Name Other Properties DataGridView   dgvEmployees DataSource: dsEmployees DataMember: Employee Button Close btnClose

    Data Grid Columns

    DataPropertyName HeaderText Width EmployeeNumber Empl # 70 FirstName First Name 65 LastName Last Name 65 Title   110 HourlySalary Salary/Hr 55 colEmployeeDetails Employee Details 150

17. Double-click an unoccupied area of the form and implement the event as follows:
    

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;
    using System.IO;
    
    namespace WattsALoan1
    {
        public partial class Employees : Form
        {
            public Employees()
            {
                InitializeComponent();
            }
    
            private void Employees_Load(object sender, EventArgs e)
            {
                string strFilename = "employees.xml";
    
                if (File.Exists(strFilename))
                    dsEmployees.ReadXml(strFilename);
            }
        }
    }

18. Return to the form and click an unoccupied area of its body
19. In the Properties window, click the Events button and double-click FormClosing
20. Implement the event as follows:
    

    private void Employees_FormClosing(object sender, FormClosingEventArgs e)
    {
        dsEmployees.WriteXml("employees.xml");
    }

21. Return to the form and double-click the Close button
22. Implement the event as follows:
    

    private void btnClose_Click(object sender, EventArgs e)
    {
        Close();
    }

23. Return to the form
24. Under the form, right-click dsEmployees and click Copy
25. Display the LoansAllocations form
26. Right-click it and click Paste
27. To create a new form, in the Solution Explorer, right-click WattsALoan1 -> Add -> Windows Forms...
28. Set the Name to Customers and click Add
29. From the Data section of the Toolbox, click DataSet and click the form
30. Select the Untyped Dataset radio button and click OK
31. In the Properties window, change the following characteristics:
    DataSetName: dsCustomers
    (Name): Customers
32. Click Tables and click its ellipsis button
33. To create a new table, click Add and change the properties as follows:
    TableName: Customer
    (Name): tblCustomer
34. Click Columns and click its ellipsis button
35. Click Add 5 times and change the properties as follows:
     

    AllowDBNull	ColumnName	Unique	Expression	(Name)
    False	AccountNumber	True		colAccountNumber
    	FirstName			colFirstName
    False	LastName			colLastName
    	EmailAddress			colEmailAddress
    	PhoneNumber			colPhoneNumber
    	CustomerDetails		AccountNumber + ': ' + FirstName + ' ' + LastName	colCustomerDetails

36. Click Close and click Close
37. Design the form as follows:
     

    Control Text Name Other Properties DataGridView   dgvCustomers DataSource: dsCustomers DataMember: Customer Button Close btnClose

    Data Grid Columns

    DataPropertyName HeaderText Width AccountNumber Account # 65 FirstName First Name 60 LastName Last Name 65 EmailAddress Email Address 120 PhoneNumber Phone # 90 CustomerDetails Customer Details 150

38. Double-click an unoccupied area of the form and implement the event as follows:
    

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;
    using System.IO;
    
    namespace WattsALoan1
    {
        public partial class Customers : Form
        {
            public Customers()
            {
                InitializeComponent();
            }
    
            private void Customers_Load(object sender, EventArgs e)
            {
                string strFilename = "customers.xml";
    
                if (File.Exists(strFilename))
                    dsCustomers.ReadXml(strFilename);
            }
        }
    }

39. Return to the form and click an unoccupied area of its body
40. In the Properties window, click the Events button and double-click FormClosing
41. Implement the event as follows:
    

    private void Customers_FormClosing(object sender, FormClosingEventArgs e)
    {
        dsCustomers.WriteXml("customers.xml");
    }

42. Return to the form and double-click the Close button
43. Implement the event as follows:
    

    private void btnClose_Click(object sender, EventArgs e)
    {
        Close();
    }

44. Return to the form
45. Under the form, right-click dsCustomers and click Copy
46. Display the LoansAllocations form
47. Right-click it and click Paste
48. Design the form as follows:

    Control Text Name Other Properties DataGridView   dgvCustomers DataSource: dsCustomers DataMember: Customer Button Close btnClose
    Data Grid Columns
    DataPropertyName HeaderText Width DefaultCellStyle -> Format   LoanNumber Loan # 60     DateAllocated Date Allocated 85 Date Time   PreparedBy Prepared By 165   ColumnType: DataGridViewComboBoxColumn DataSource: dsEmployees DisplayMember: Employee.EmployeeDetails PreparedFor Prepared For 165   ColumnType: DataGridViewComboBoxColumn DataSource: dsCustomers DisplayMember: Customer.CustomerDetails Principal   65 Currency   InterestRate Rate (%) 55 Numeric   Periods Prd (Months) 75 Numeric   InterestEarned Interest Earned 85 Currency   FutureValue Future Value 75 Currency   MonthlyPayment Pmt/Month 70 Currency

49. Double-click an unoccupied area of the form and implement the event as follows:
    

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;
    using System.IO;
    
    namespace WattsALoan1
    {
        public partial class LoanAllocations : Form
        {
            public LoanAllocations()
            {
                InitializeComponent();
            }
    
            private void LoanAllocations_Load(object sender, EventArgs e)
            {
                string strFilename = "employees.xml";
    
                if (File.Exists(strFilename))
                    dsEmployees.ReadXml(strFilename);
    
                strFilename = "customers.xml";
    
                if (File.Exists(strFilename))
                    dsCustomers.ReadXml(strFilename);
    
                strFilename = "loans.xml";
    
                if (File.Exists(strFilename))
                    dsLoansAllocations.ReadXml(strFilename);
            }
        }
    }

50. Return to the form and click an unoccupied area of its body
51. In the Properties window, click the Events button and double-click FormClosing
52. Implement the event as follows:
    

    private void LoanAllocations_FormClosing(object sender, FormClosingEventArgs e)
    {
        dsLoansAllocations.WriteXml("loans.xml");
    }

53. Return to the form and double-click the Close button
54. Implement the event as follows:
    

    private void btnClose_Click(object sender, EventArgs e)
    {
        Close();
    }

55. To create a new form, in the Solution Explorer, right-click WattsALoan1 -> Add -> Windows Forms...
56. Set the Name to Payments and click Add
57. Display the Employees form
58. Right-click dsEmployees and click Copy
59. Display the Payments form
60. Right-click it and click Paste
61. Display the LoanAllocations form
62. Right-click dsLoanAllocations and click Copy
63. Display the Payments form
64. Right-click it and click Paste
65. From the Data section of the Toolbox, click DataSet and click the Payments form
66. Select the Untyped Dataset radio button and click OK
67. In the Properties window, change the following characteristics:
    DataSetName: dsPayments
    (Name): Payments
68. Click Tables and click its ellipsis button
69. To create a new table, click Add and change the properties as follows:
    TableName: Payment
    (Name): tblPayment
70. Click Columns and click its ellipsis button
71. Click Add 6 times and change the properties as follows:
     

    AllowDBNull	ColumnName	DataType	DefaultValue	Expression	Unique	(Name)
    False	PaymentNumber				True	colPaymentNumber
    False	PaymentDate	System.DateTime				colPaymentDate
    False	ReceivedBy					colReceivedBy
    False	PaymentFor					colPaymentFor
    False	PaymentAmount	System.Double	0.00			colPaymentAmount
    	Balance	System.Double	0.00			colBalance

72. Click Close and click Close
73. Design the form as follows:
     

    Control Text Name Other Properties DataGridView   dgvPayments DataSource: dsPayments DataMember: Payment Button Loans... btnLoans   Button Close btnClose

    Data Grid Columns

    DataPropertyName HeaderText Width DefaultCellStyle -> Format   PaymentNumber Pmt # 55     PaymentDate Pmt Date 70 Date Time   ProcessedBy Processed By     ColumnType: DataGridViewComboBoxColumn DataSource: dsEmployees DisplayMember: Employee.EmployeeNumber PaymentFor Payment For     ColumnType: DataGridViewComboBoxColumn DataSource: dsLoansAllocations DisplayMember: Loan.LoanNumber PaymentAmount Pmt Amt 70 Currency   Balance   80 Currency

74. Double-click an unoccupied area of the form and implement the event as follows:
    

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Windows.Forms;
    using System.IO;
    
    namespace WattsALoan1
    {
        public partial class Payments : Form
        {
            public Payments()
            {
                InitializeComponent();
            }
    
            private void Payments_Load(object sender, EventArgs e)
            {
                string strFilename = "loans.xml";
    
                if (File.Exists(strFilename))
                    dsLoansAllocations.ReadXml(strFilename);
    
                strFilename = "employees.xml";
    
                if (File.Exists(strFilename))
                    dsEmployees.ReadXml(strFilename);
    
                strFilename = "payments.xml";
    
                if (File.Exists(strFilename))
                    dsPayments.ReadXml(strFilename);
            }
        }
    }

75. Return to the form and click an unoccupied area of its body
76. In the Properties window, click the Events button and double-click FormClosing
77. Implement the event as follows:
    

    private void Payments_FormClosing(object sender, FormClosingEventArgs e)
    {
        dsPayments.WriteXml("payments.xml");
    }

78. Return to the form and double-click the Loans button
79. Implement the event as follows:
    

    private void btnLoans_Click(object sender, EventArgs e)
    {
        LoanAllocations frmLoans = new LoanAllocations();
        frmLoans.ShowDialog();
    }

80. Return to the form and double-click the Close button
81. Implement the event as follows:
    

    private void btnClose_Click(object sender, EventArgs e)
    {
        Close();
    }

82. In the Solution Explorer, right-click Form1.cs and click Rename
83. Type WattsALoan.cs and press Enter twice
84. Display the form and cesign it as follows:
     

    Control Text Name Font Button ... btnPayments   Label Loan Payments lblPayments Garamond, Bold, 24 Button ... btnAllocations   Label Loan Allocations lblAllocations Garamond, Bold, 24 Button ... btnCustomers   Label Customers lblCustomers Garamond, Bold, 24 Button ... btnEmployees   Label Employees lblEmployees Garamond, Bold, 24 Button Close btnClose

85. Double-click the top button and implement its event as follows:
    

    private void btnPayments_Click(object sender, EventArgs e)
    {
        Payments frmPayments = new Payments();
        frmPayments.ShowDialog();
    }

86. Return to the form and click the Loan Payments label
87. In the Properties window, click Events, then click the arrow of Click and select btnPayments_Click
88. On the form, double-click the second button from top
89. Implement its event as follows:
    

    private void btnLoanAllocations_Click(object sender, EventArgs e)
    {
        LoansAllocations frmLoans = new LoansAllocations();
        frmLoans.ShowDialog();
    }

90. Return to the form and click the Loan Allocations label
91. In the Events section of the Properties window, click the arrow of Click and select btnLoanAllocations_Click
92. On the form, double-click the third button from top
93. Implement its event as follows:
    

    private void btnCustomers_Click(object sender, EventArgs e)
    {
        Customers frmClients = new Customers();
        frmClients.ShowDialog();
    }

94. Return to the form and click the Customers label
95. In the Events section of the Properties window, click the arrow of Click and select btnCustomers_Click
96. On the form, double-click the fourth button from top
97. Implement its event as follows:
    

    private void btnEmployees_Click(object sender, EventArgs e)
    {
        Employees frmStaff = new Employees();
        frmStaff.ShowDialog();
    }

98. In the Events section of the Properties window, click the arrow of Click and select btnEmployees_Click
99. Return to the form and double-click the Close button
100. Implement its event as follows:
     

     private void btnClose_Click(object sender, EventArgs e)
     {
         Close();
     }

101. Execute the application
102. Click the Employees label and create the following records:
      

     Employee #	First Name	Last Name	Title	Salary/hr
     7973-45	Bernard	Wallow	Account Manager	24.85
     2497-94	Justine	Bogley	Sales Representative	12.75
     2930-75	Nestor	Rosenblatt	Sales Representative	14.25

      
     
103. Close the form
104. Click the Customers label and create the following records:
      

     Account #	First Name	Last Name	Email Address	Phone Number
     937-497	Joan	Fairbanks	fairbie1288@hotmail.com	(301) 937-5888
     293-759	Ernie	Lipps	ernie.rowdie@comcast.net	(703) 506-0000
     502-850	Christopher	Owens	owenchris@yahoo.com	(202) 529-6100
     520-840	Ann	Rowdy	rowdiant@msn.com	(301) 855-2090
     602-475	Sarah	Thompson	lolitta448@yahoo.com	(301) 870-7454

      
     
105. Close the form
106. Click the Loan Allocations label and create the following records:
      

     Loan #	Date Allocated	Prepared By	Prepared For	Principal	Rate (%)	Prd (Months)
     52-9739-5	08/18/14	2497-94	937-497	6500	16.25
     20-5804-8	10/26/2014	7973-45	602-475	3260
     77-3907-2	02/06/15	2497-94	502-850	25605	12.50	60
     92-7495-4	03/20/15	2930-75	293-759	14800		48

     

107. Close the form
108. Click the Loan Payments label and create the following records:
      

     Pmt #	Pmt Date	Received By	Payment For	Pmt Amt	Balance
     1001	10/25/14	2497-94	52-9739-5	268.58	9400.17
     1002	11/30/14	2930-75	52-9739-5	268.58	9131.59
     1003	12/24/2014	7973-45	20-5804-8	114.33	4001.42
     1004	12/28/14	2497-94	52-9739-5	268.58	8863.01
     1005	01/26/15	2497-94	20-5804-8	114.33	3887.09
     1006	01/31/15	2930-75	52-9739-5	268.58	8594.43
     1007	02/20/15	2497-94	20-5804-8	114.33	3772.76
     1008	03/02/15	2930-75	52-9739-5	268.58	8325.85
     1009	03/25/2015	2930-75	20-5804-8	114.33	3658.43
     1010	04/25/15	7973-45	92-7495-4	416.25	19563.75
     1011	04/28/15	2497-94	77-3907-2	693.47	40914.66
     1012	04/28/15	7973-45	20-5804-8	114.33	3544.10
     1013	05/01/15	7973-45	52-9739-5	268.58	8057.27
     1014	05/26/15	2497-94	77-3907-2	693.47	40221.19

      
109. Close the forms and return to your programming environment