I'm finishing up my customer membership provider and seem to be stuck on this error that I have thrown. It doesn't seem to be grabbing the connection string properly. I have rebuilt the provider several times to no avail. Is there anything special I am missing. I have tried to login, but the ProviderException is thrown every time. Don't mind the names, I removed the company name from them.

Web.config

1    <membership defaultProvider="MembershipProvider">
2          <providers>
3            <clear />
4            <add name="StaplesMemberShipProvider" 
5                type="MemberShipProvider.MembershipProvider"
6                connectionStringName="ConnectionStringTest"
7                enablePasswordRetrieval="true"
8                enablePasswordReset="true"
9                requiresQuestionAndAnswer="false" />
10         </providers>
11       </membership>

MembershipProvider.vb

1    Public Overrides Sub Initialize(ByVal name As String, ByVal config As NameValueCollection)
2            ...    
3            Dim ConnectionStringSettings As ConnectionStringSettings = ConfigurationManager.ConnectionStrings(config("ConnectionStringTest"))
4    
5            If ConnectionStringSettings Is Nothing OrElse ConnectionStringSettings.ConnectionString.Trim() = String.Empty Then
6                Throw New ProviderException("Connection string cannot be blank.")
7            End If
8    
9            _sqlConnectionString = ConnectionStringSettings.ConnectionString
10   
11   End Sub

 Please let us see your full web.config

Have you put connection string

<connectionStrings>
        <add name="ConnectionStringTest" connectionString="server=ty\yukon;database=5656;UID=sa;PWD=sa;Pooling=false" providerName="System.Data.SqlClient"/>
    </connectionStrings>

here is my full Web.config

1    <?xml version="1.0"?>
2    <configuration>
3    	<configSections>
4    		<sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
5    			<sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
6    				<section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication"/>
7    				<sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35">
8    					<section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="Everywhere"/>
9    					<section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication"/>
10   					<section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication"/>
11   					<section name="roleService" type="System.Web.Configuration.ScriptingRoleServiceSection, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" requirePermission="false" allowDefinition="MachineToApplication"/></sectionGroup>
12   			</sectionGroup>
13   		</sectionGroup>
14   	</configSections>
15   	<connectionStrings>
16       <!--<add name="ConnectionString" connectionString="Data Source=XXXX.XXXX.com;Initial Catalog=XXXX;User ID=XXXX;Password=XXXX;" providerName="System.Data.SqlClient" />-->
17       <add name="ConnectionStringTest" connectionString="Data Source=XXXX.XXXX.com;Initial Catalog=XXXX;User ID=XXXX;Password=XXXX" providerName="System.Data.SqlClient" />
18    </connectionStrings>
19    <system.web>
20   		<pages>
21   			<controls>
22   				<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
23   				<add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="ajaxToolkit"/>
24   				<add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/></controls>
25   		</pages>
26   
27       <authentication mode="Forms">
28         <forms defaultUrl="login.aspx" name=".ASPSTPAUTH"></forms>
29       </authentication>
30       <authorization>
31         <deny users="?" />
32       </authorization>
33   
34      <machineKey
35         validationKey="C50B3C89CB21F4F1422FF158A5B42D0E8DB8CB5CDA1742572A487D9401E3400267682B202B746511891C1BAF47F8D25C07F6C39A104696DB51F17C529AD3CABE"
36         decryptionKey="8A9BE8FD67AF6979E7D20198CFEA50DD3D3799C77AF2B72F"
37         validation="SHA1" />
38      
39       <!-- Customer Membership Provider Begin -->
40       <membership defaultProvider="MembershipProvider">
41         <providers>
42           <clear />
43           <add name="MemberShipProvider" 
44               type="MemberShipProvider.MembershipProvider"
45               connectionStringName="ConnectionStringTest"
46               enablePasswordRetrieval="true"
47               enablePasswordReset="true"
48               requiresQuestionAndAnswer="false" />
49         </providers>
50       </membership>
51       <!-- Customer Membership Provider End -->
52      
53   		<!--
54             Set compilation debug="true" to insert debugging
55             symbols into the compiled page. Because this
56             affects performance, set this value to true only
57             during development.
58       -->
59      <customErrors mode="Off" />
60      
61   		<compilation debug="true">
62   			<assemblies>
63   				<add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
64   				<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
65   				<add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
66   				<add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
67   				<add assembly="System.Data.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/></assemblies></compilation>
68   		<httpHandlers>
69   			<remove verb="*" path="*.asmx"/>
70   			<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
71   			<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
72   			<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" validate="false"/>
73   		</httpHandlers>
74   		<httpModules>
75   			<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
76   		</httpModules>
77   	</system.web>
78    <system.webServer>
79   		<validation validateIntegratedModeConfiguration="false"/>
80   		<modules>
81   			<remove name="ScriptModule"/><add name="ScriptModule" preCondition="managedHandler" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
82   		</modules>
83   		<handlers>
84   			<remove name="ScriptHandlerFactory"/>
85   			<remove name="ScriptHandlerFactoryAppServices"/>
86   			<remove name="ScriptResource"/><remove name="WebServiceHandlerFactory-Integrated"/>
87   			<add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
88   			<add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
89   			<add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
90   		</handlers>
91   	</system.webServer>
92   	<system.codedom>
93   		<compilers>
94   			<compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
95   				<providerOption name="CompilerVersion" value="v3.5"/>
96   				<providerOption name="WarnAsError" value="false"/></compiler>
97   			<compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.VisualBasic.VBCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
98   				<providerOption name="CompilerVersion" value="v3.5"/>
99   				<providerOption name="OptionInfer" value="true"/>
100  				<providerOption name="WarnAsError" value="false"/></compiler></compilers></system.codedom>
101  	<runtime>
102  		<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
103  			<dependentAssembly>
104  				<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
105  				<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/></dependentAssembly>
106  			<dependentAssembly>
107  				<assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>
108  				<bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/></dependentAssembly></assemblyBinding></runtime></configuration>
109  

Change

Dim ConnectionStringSettings As ConnectionStringSettings = ConfigurationManager.ConnectionStrings(config("ConnectionStringTest"))

to

Dim ConnectionStringSettings As ConnectionStringSettings = ConfigurationManager.ConnectionStrings("ConnectionStringTest") 

After a few minutes looking at it I noticed for some reason I had changed the static text in the line below to the actual name of the Connection string.

What I had 

Dim ConnectionStringSettings As ConnectionStringSettings = ConfigurationManager.ConnectionStrings(config("ConnectionStringTest"))

Dim ConnectionStringSettings As ConnectionStringSettings = ConfigurationManager.ConnectionStrings(config("connectionStringName")) - this references the connectionStringName in the provider

1   <membership defaultProvider="MembershipProvider">
2          <providers>
3            <clear />
4            <add name="StaplesMemberShipProvider"
5                type="MemberShipProvider.MembershipProvider"
6                connectionStringName="ConnectionStringTest"
7                enablePasswordRetrieval="true"
8                enablePasswordReset="true"
9                requiresQuestionAndAnswer="false" />
10         </providers>
11       </membership>