How to iterate over dependencies of a project in gradle

I have defined a simple task as

 task printCompileTestLib {
      configurations.testCompile.each { println it }
 }

 test.dependsOn(printCompileTestLib)

But when I execute the task following error happens:

FAILURE: Build failed with an exception.

* Where:
Build file 'D:\projects\fwk\trunk-gradle\ext\build.gradle' line: 2

* What went wrong:
A problem occurred evaluating project ':ext'.
> You can't change a configuration which is not in unresolved state!

* Try:
Run with --debug option to get more log output.

* Exception is:
org.gradle.api.GradleScriptException: A problem occurred evaluating project ':ext'.
    at org.gradle.groovy.scripts.internal.DefaultScriptRunnerFactory$ScriptRunnerImpl.run(DefaultScriptRunnerFactory.java:54)
    at org.gradle.configuration.DefaultScriptPluginFactory$ScriptPluginImpl.apply(DefaultScriptPluginFactory.java:132)
    at org.gradle.configuration.project.BuildScriptProcessor.execute(BuildScriptProcessor.java:38)
    at org.gradle.configuration.project.BuildScriptProcessor.execute(BuildScriptProcessor.java:25)
    at org.gradle.configuration.project.ConfigureActionsProjectEvaluator.evaluate(ConfigureActionsProjectEvaluator.java:34)
    at org.gradle.configuration.project.LifecycleProjectEvaluator.evaluate(LifecycleProjectEvaluator.java:55)
    at org.gradle.api.internal.project.AbstractProject.evaluate(AbstractProject.java:468)
    at org.gradle.api.internal.project.AbstractProject.evaluate(AbstractProject.java:76)
    at org.gradle.configuration.DefaultBuildConfigurer.configure(DefaultBuildConfigurer.java:31)
    at org.gradle.initialization.DefaultGradleLauncher.doBuildStages(DefaultGradleLauncher.java:142)
    at org.gradle.initialization.DefaultGradleLauncher.doBuild(DefaultGradleLauncher.java:113)
    at org.gradle.initialization.DefaultGradleLauncher.run(DefaultGradleLauncher.java:81)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter$DefaultBuildController.run(InProcessBuildActionExecuter.java:64)
    at org.gradle.launcher.cli.ExecuteBuildAction.run(ExecuteBuildAction.java:33)
    at org.gradle.launcher.cli.ExecuteBuildAction.run(ExecuteBuildAction.java:24)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter.execute(InProcessBuildActionExecuter.java:35)
    at org.gradle.launcher.exec.InProcessBuildActionExecuter.execute(InProcessBuildActionExecuter.java:26)
    at org.gradle.launcher.cli.RunBuildAction.run(RunBuildAction.java:50)
    at org.gradle.api.internal.Actions$RunnableActionAdapter.execute(Actions.java:171)
    at org.gradle.launcher.cli.CommandLineActionFactory$ParseAndBuildAction.execute(CommandLineActionFactory.java:201)
    at org.gradle.launcher.cli.CommandLineActionFactory$ParseAndBuildAction.execute(CommandLineActionFactory.java:174)
    at org.gradle.launcher.cli.CommandLineActionFactory$WithLogging.execute(CommandLineActionFactory.java:170)
    at org.gradle.launcher.cli.CommandLineActionFactory$WithLogging.execute(CommandLineActionFactory.java:139)
    at org.gradle.launcher.cli.ExceptionReportingAction.execute(ExceptionReportingAction.java:33)
    at org.gradle.launcher.cli.ExceptionReportingAction.execute(ExceptionReportingAction.java:22)
    at org.gradle.launcher.Main.doAction(Main.java:46)
    at org.gradle.launcher.bootstrap.EntryPoint.run(EntryPoint.java:45)
    at org.gradle.launcher.Main.main(Main.java:37)
    at org.gradle.launcher.bootstrap.ProcessBootstrap.runNoExit(ProcessBootstrap.java:50)
    at org.gradle.launcher.bootstrap.ProcessBootstrap.run(ProcessBootstrap.java:32)
    at org.gradle.launcher.GradleMain.main(GradleMain.java:23)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: org.gradle.api.InvalidUserDataException: You can't change a configuration which is not in unresolved state!
    at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.throwExceptionIfNotInUnresolvedState(DefaultConfiguration.java:425)
    at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration.access$500(DefaultConfiguration.java:45)
    at org.gradle.api.internal.artifacts.configurations.DefaultConfiguration$VetoContainerChangeAction.run(DefaultConfiguration.java:525)
    at org.gradle.api.internal.DefaultDomainObjectCollection.assertMutable(DefaultDomainObjectCollection.java:258)
    at org.gradle.api.internal.DefaultDomainObjectCollection.add(DefaultDomainObjectCollection.java:158)
    at org.gradle.api.internal.DelegatingDomainObjectSet.add(DelegatingDomainObjectSet.java:80)
    at org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.doAdd(DefaultDependencyHandler.groovy:66)
    at org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.this$2$doAdd(DefaultDependencyHandler.groovy)
    at org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.methodMissing(DefaultDependencyHandler.groovy:102)
    at org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.invokeMethod(DefaultDependencyHandler.groovy)
    at build_1baeie9kemtfk46808kipbh9q9$_run_closure1.doCall(D:\projects\fwk\trunk-gradle\ext\build.gradle:2)
    at org.gradle.api.internal.ClosureBackedAction.execute(ClosureBackedAction.java:58)
    at org.gradle.util.ConfigureUtil.configure(ConfigureUtil.java:130)
    at org.gradle.util.ConfigureUtil.configure(ConfigureUtil.java:91)
    at org.gradle.api.internal.project.AbstractProject.dependencies(AbstractProject.java:883)
    at org.gradle.api.internal.BeanDynamicObject$MetaClassAdapter.invokeMethod(BeanDynamicObject.java:246)
    at org.gradle.api.internal.BeanDynamicObject.invokeMethod(BeanDynamicObject.java:134)
    at org.gradle.api.internal.CompositeDynamicObject.invokeMethod(CompositeDynamicObject.java:147)
    at org.gradle.groovy.scripts.BasicScript.methodMissing(BasicScript.java:79)
    at build_1baeie9kemtfk46808kipbh9q9.run(D:\projects\fwk\trunk-gradle\ext\build.gradle:1)
    at org.gradle.groovy.scripts.internal.DefaultScriptRunnerFactory$ScriptRunnerImpl.run(DefaultScriptRunnerFactory.java:52)
    ... 31 more

The question is how can I iterate over dependencies of a project? Is there any way to extract each dependency's group, module and version in a task?

You need to iterate over the dependencies at task execution time, rather than at task configuration time:

task printCompileTestLib {
    // doLast adds a so-called task action, which will 
    // only get run if and when the task gets executed
    doLast { 
        configurations.testCompile.each { println it }
    }
}