read string from .resx file in C#

ResourceManager shouldn't be needed unless you're loading from an external resource.
For most things, say you've created a project (DLL, WinForms, whatever) you just use the project namespace, "Resources" and the resource identifier. eg:

Assuming a project namespace: UberSoft.WidgetPro

And your resx contains:

You can just use:

Ubersoft.WidgetPro.Properties.Resources.RESPONSE_SEARCH_WILFRED

This example is from the MSDN page on ResourceManager.GetString():

// Create a resource manager to retrieve resources.
ResourceManager rm = new ResourceManager("items", Assembly.GetExecutingAssembly());

// Retrieve the value of the string resource named "welcome".
// The resource manager will retrieve the value of the  
// localized resource using the caller's current culture setting.
String str = rm.GetString("welcome");

Try this, works for me.. simple

Assume that your resource file name is "TestResource.resx", and you want to pass key dynamically then,

string resVal = TestResource.ResourceManager.GetString(dynamicKeyVal);

Add Namespace

using System.Resources;