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POObject reference not set to an instance of an object when variable is declared in class level
 

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    <p>Sometimes get this error "Object reference not set to an instance of an object" when I declare a variable in class level, and use that variable in a procedure.</p> <p>For example, this is my latest project to create a program to display the state of substances in a specific temperature:</p> <pre><code> Public Class Form1 Const WATER_FP As Integer = 0 Const WATER_BP As Integer = 100 Const ETHANOL_FP As Integer = -114 Const ETHANOL_BP As Integer = 78 Const MERCURY_FP As Integer = -39 Const MERCURY_BP As Integer = 357 Const OXYGEN_FP As Integer = -219 Const OXYGEN_BP As Integer = -183 Dim strSolid As String = vbNullString Dim strGas As String = vbNullString Dim sngTemperature As Single = CSng(txtTemperature.Text) Private Sub btnDisplayState_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplayState.Click If sngTemperature &lt;= WATER_FP Then strSolid &amp;= "Water " ElseIf sngTemperature &gt;= WATER_BP Then strGas &amp;= "Water " End If If sngTemperature &lt;= ETHANOL_FP Then strSolid &amp;= "Ethanol " ElseIf sngTemperature &gt;= ETHANOL_BP Then strGas &amp;= "Ethanol " End If If sngTemperature &lt;= MERCURY_FP Then strSolid &amp;= "Mercury " ElseIf sngTemperature &gt;= MERCURY_BP Then strGas &amp;= "Mercury " End If If sngTemperature &lt;= OXYGEN_FP Then strSolid &amp;= "Oxygen " ElseIf sngTemperature &gt;= OXYGEN_BP Then strGas &amp;= "Oxygen " End If If strSolid &lt;&gt; vbNullString Then strSolid = "Substances that are in solid state are " &amp; strSolid Else strSolid = "No substances are in solid state." End If If strGas &lt;&gt; vbNullString Then strGas = "Substances that are in gaseous state are " &amp; strGas Else strGas = "No substances are in gaseous state." End If lblMessage.Text = strSolid &amp; ControlChars.CrLf &amp; strGas End Sub End Class </code></pre> <p>Upon debugging, I get an error as described above, without any highlight of where the code is causing the problem. However, if I move the variables into <code>btnDisplayState_Click</code>, the program works perfectly. Now I know what caused the error, but furthermore I wanted to know why this problem occurs.</p>
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