StructureMap – Configuring Instances

One of the most common tasks in the Registry DSL is defining how a objects will be created by configuring Instances.  While you can create a custom Instance class, out of the box StructureMap provides all of the most common scenarios — and some that probably aren’t that common.  See Using Open Generic Types for more specific information about configuring open generic types.

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Instance Expression

In an effort to standardize the Registry DSL and make the API more predictable and consistent, we have introduced the “InstanceExpression” as a shared Expression Builder that is used in every place that the Registry DSL needs to define an Instance.  Here are several examples of how and when the InstanceExpression is invoked.  In the code sample below, the text “**********;” represents a call to an InstanceExpression.

public class InstanceExampleRegistry : Registry
public InstanceExampleRegistry()
// Set the default Instance of a PluginType
// Add an additional Instance of a PluginType
// Add multiple additional Instances of a PluginType
ForRequestedType<IService>().AddInstances(x =>
// Use the InstanceExpression to define the default Instance
// of a PluginType within a Profile
CreateProfile("Connected", x =>


InstanceExpression Operations

As of 2.5+, all operations in the Registry DSL that require the definition of an Instance expose an option to use an InstanceExpression with these options:

public interface IInstanceExpression<T> : span style="color: #2b91af;">IsExpression<T>
// Attach an Instance object that is configured
/// independently of the DSL.  This is an extensibility point
void Instance(Instance instance);
void IsThis(Instance instance);
// Use a pre-built object
LiteralInstance IsThis(T obj);
LiteralInstance Object(T theObject);
// Use a type and build with its constructor function
SmartInstance<PLUGGEDTYPE> OfConcreteType<PLUGGEDTYPE>() where PLUGGEDTYPE : T;
ConfiguredInstance OfConcreteType(Type type);
// Build by a Lambda or an Anonymous Delegate
ConstructorInstance<T> ConstructedBy(Func<T> func);
ConstructorInstance<T> ConstructedBy(Func<IContext, T> func);
// Refer to a named Instance
ReferencedInstance TheInstanceNamed(string key);
DefaultInstance TheDefault();
// Use a cloned copy of the template
PrototypeInstance PrototypeOf(T template);
// Cache the template as a binary serialized blob
SerializedInstance SerializedCopyOf(T template);
// Load an ASCX control
UserControlInstance LoadControlFrom(string url);

New for StructureMap 2.5.2+ is the “Conditional” construction choice.

// Conditional object construction
ConditionalInstance<T> Conditional(Action<ConditionalInstance<T>.ConditionalInstanceExpression&<T>> configuration);

Building with Constructors and Setters

We received a lot of feedback on the Registry DSL introduced in 2.0.  Because of that feedback, StructureMap 2.5+ contains a new feature called the SmartInstance that is just a better Fluent Interface for specifying constructor and setter arguments and dependencies of a concrete type.  The main complaint with the 2.0 API was that SetProperty() is overloaded to mean either setter or constructor arguments.  The underlying mechanisms in StructureMap stores the information the same way, but the API was causing real confusion.  The SmartInstance API is designed to eliminate the confusion over constructor vs setter arguments, and also to take advantage of newer .Net 3.5 features./p>

Designating the Type

The first step is to define the actual concrete type of this Instance by using the OfConcreteType<T>() method on InstanceExpression to start the definition of a SmartInstance.

// Add an additional Instance of a PluginType


Defining primitive constructor arguments

Primitive arguments like strings or value types (including enumerations) are defined with the WithCtorArg(name) expression.  In this example below, the Thing class has several constructor arguments.

public Thing(int count, string name, double average, Rule rule)
_count = count;
_name = name;
_average = average;
_rule = rule;

In order to make a SmartInstance for the “Thing” type, I need to specify values for all of the simple constructor arguments (“count”, “name”, and “average”).  That syntax is shown in the sample below taken from a unit test:

public void DeepInstanceTest_with_SmartInstance()
assertThingMatches(registry =>


Defining primitive setter properties

You have additional options for setting a primitive setter property.  The simplest approach is to just use a Lambda expression that will be applied to the object as soon as it’s built.  Intellisense and compiler safety are good things, so you might as well use it.  StructureMap now supports optional setter injection, meaning that you no longer need to do the [Setter] attributes in the concrete classes.  If you specify the value of a setter, StructureMap will use that value regardless of whether or not the [Setter] property exists.  The same rule applies to non-primitive setter dependencies.

Let’s say I have a class like this:

public class SimplePropertyTarget
public string Name { get; set; }
public int Age { get; set; }

To set the setter properties in the configuration you can use the SetProperty(Action<T>) expression like this to set as many properties as you need:

var container = new Container(x =>
.SetProperty(target =>
target.Name = "Max";
target.Age = 4;


Overriding Constructor Dependencies

When you want to override the auto wiring behavior, you can explicitly specify the constructor dependency of an Instance by using the CtorDependency<T>() expression:

var theContainer = new Container(registry =>
.CtorDependency<IWidget>().Is(i =>

Or, if a concrete type should have two or more dependencies of the same type, you can also specify the name of the constructor argument:

.CtorDependency<IWidget>("widget").Is(x => x.TheInstanceNamed("Purple"));

The Lambda expression inside of “CtorDependency().Is()” is another InstanceExpression, so all the normal options for defining an Instance are available.  Please note that I just specified the type of constructor argument in the call to CtorDependency<T>() above.  If you have more than one constructor argument of the same type, you’ll need to specify the name of the constructor argument with the CtorDependency<T>(argumentName) method.

Overriding Setter Dependencies

Setter dependencies are configured much like constructor dependencies:

registry.ForRequestedType<Thing>().Use<span style="color: #2b91af;">Thing>()
.SetterDependency<Rule>().Is(x =>
c => c.OfConcreteType<ColorWidget>().WithCtorArg("color").EqualTo("yellow"));
&                    });

The only difference is that Setter dependencies can also be specified by using an Expression to declare the property to be injected:

var container = new Container(x =>
.SetterDependency<IWidget>(o => o.Widget).Is(o => o.Object(new ColorWidget("Red")));


Array or Non Primitive Dependencies

StructureMap supports Array arguments of non primitive objects.  An array of dependencies or type T can be defined with the TheArrayOf<T>() method shown below.

var container = new Container(x =>/p>
.TheArrayOf<IHandler>().Contains(y =>

TheArrayOf<T>() method is another Nested Closure that takes in an Action<IInstanceExpression> to configure the child Instance’s in the array.

Primitive Arrays and Dictionaries

Arrays of primitive types (strings, numbers, etc) and Dictionary types can be configured just by treating them as a child dependency that is configured inline.  Let’s say you have a class like this that needs a couple Array objects in its constructor function:

public class ClassWithStringAndIntArray
private readonly int[] _numbers;
private readonly string[] _strings;
public ClassWithStringAndIntArray(int[] numbers, string[] strings)
_numbers = numbers;
_strings = strings;
public int[] Numbers
get { return _numbers; }
public string[] Strings
get { return _strings; }

You can configure the values for the “numbers” and “strings” constructor function arguments like this:

public void specify_a_string_array()
var container = new Container(x =>
.CtorDependency<string[]>().Is(new[] {"a", "b", "c"})
.CtorDependency<int[]>().Is(new[] {1, 2, 3});
var objectWithArrays = container.GetInstance<ClassWithStringAndIntArray>();
objectWithArrays.Numbers.ShouldEqual(new[] {1, 2, 3});
objectWithArrays.Strings.ShouldEqual(new[] {"a", "b", "c"});

Using a Custom Instance

Occasionally, it might be advantageous to create a custom form of Instance.  Fortunately, there’s a mechanism in the InstanceExpression to attach Instance objects built independently of the Registry DSL.  Let’s say that you’ve created a custom Instance called XmlFileInstance<T> that deserializes an object from an Xml file.  The registration of that custom Instance might look something like this:

public class CustomInstanceRegistry : Registry { public CustomInstanceRegistry() { ForRequestedType<Address>().TheDefault.IsThis(new XmlFileInstance<Address>("address1.xml")); } }

See Extending StructureMap for more specific information on using custom Instance classes.

Using an Externally Constructed Object

It’s frequently useful to simply inject an existing object into StructureMap.  This frequently arises in desktop applications where parts of the application shell may be created independently of the container, but you still with these objects to be injected into other services.  Here’s an example from my current project where we need to programmatically build up some services (IUrlRegistry, IActionRegistry, and ITypeRegistry) before injecting them into StructureMap:

private readonly TypeRegistry _typeRegistry = new TypeRegistry(); private UrlGraph _graph; private ControllerRegistry _registry; public WebRegistry(ICollection<RouteBase> routes) { createUrlGraph(routes); ForRequestedType<IUrlRegistry>().TheDefault.Is.Object(_graph); ForRequestedType<IActionRegistry>().TheDefault.Is.Object(_actionRegistry); ForRequestedType<ITypeRegistry>().TheDefault.Is.Object(_typeRegistry); }

The Object() method is the pertinant method in the InstanceExpression.  There is also an alternative syntax called “IsThis”:

var container = new Container(x => { x.ForRequestedType<ClassWithDependency>().Use<ClassWithDependency>() .TheArrayOf<Rule>().Contains(arr => { arr.IsThis(new ColorRule("Red")); }); });


Choosing “IsThis” or “Object” is strictly a matter of aesthetics.

Also see Changing Configuration at Runtime for some related behavior.

Constructing Objects with Lambda Functions

Some classes simply cannot be created with constructor functions.  Other objects may need to be located by invoking some sort of other infrastructure (the built in Provider model in ASP.Net 2.0+ is a good example of this).  In those cases, StructureMap can instead build or find these objects by registering a Lambda expression with the ConstructedBy() method of InstanceExpression.

Let’s say that your system includes an important legacy class named WeirdLegacyRepository that manages its own lifecycle somehow, and new instances can only be accessed by a static method on the WeirdLegacyRepository.  You still want to inject these objects into the classes that depend on WeirdLegacyRepository, so you might do something like this code below:

ForRequestedType<IRepository>().TheDefault.Is.ConstructedBy(() => WeirdLegacyRepository.Current).WithName("Weird");

That example used a no argument Lambda function.  You can also register a Lambda expression that uses the current BuildSession/IContext to access other services:

ForRequestedType<ISession>().TheDefault.Is.ConstructedBy( context => context.GetInstance<ISessionSource>().CreateSession()); ForRequestedType<ITransaction>().TheDefault.Is.ConstructedBy( context => context.GetInstance<ISession>().Transaction);

See Using the Session Context for more information.

Referencing a Named Instance

Many times it is helpful to simply say “use the Instance with this name here” when configuring array dependencies or overriding the autowiring defaults.  Use the TheInstanceNamed() function as shown below to do this:

IContainer manager = new Container(r =>
.TheArrayOf<IHandler>().Contains(x =>

Using the Default Instance

Sometimes you may want to just tell StructureMap to use the default Instance of a type.  The most common usage of this is telling StructureMap to inject an optional setter property.  Use the “IsTheDefault()” or “TheDefault()” method to force the injection of a setter on an Instance by Instance basis.

[Test] public void AutoFill_a_property() { var container = new Container(r => { r.ForConcreteType<ClassWithDependency>().Configure .SetterDependency<Rule>().IsTheDefault(); r.ForRequestedType<Rule>().TheDefault.Is.Object(new ColorRule("Green")); }); container.GetInstance<ClassWithDependency>().Rule.ShouldBeOfType(typeof (ColorRule)); }

While this functionality will be supported in the future, it may be better to use the new Setter Injection Policies.

Specifying a Prototype Object with Cloning

You can have StructureMap construct objects by having it simply clone a supplied template object:

// Build an instance for IWidget, then setup StructureMap to return cloned instances of the // "Prototype" (GoF pattern) whenever someone asks for IWidget named "Jeremy" var theWidget = new CloneableWidget("Jeremy"); container = new Container(x => { x.InstanceOf<IWidget>().Is.PrototypeOf(theWidget).WithName("Jeremy"); });

There is also the option to use binary serialization as the “cloning” method.  The old advice from Microsoft is to use cloning for flat objects, and favor serialization copying for objects with lots of children.

Specifying a Prototype Object with Serialization

You can have StructureMap construct objects by having it simply clone a supplied template object by using binary serialization:

// Build an instance for IWidget, then setup StructureMap to return cloned instances of the // "Prototype" (GoF pattern) whenever someone asks for IWidget named "Jeremy" var theWidget = new CloneableWidget("Jeremy"); container = new Container(x => { x.InstanceOf<IWidget>().Is.SerializedCopyOf(theWidget).WithName("Jeremy"); });

Configuring Conditional Construction

There have been several questions on the StructureMap users list about doing conditional construction (i.e., return this object if this condition, else this other object).  In order to meet this apparent need, StructureMap 2.5.2 introduces the new ConditionalInstance that allows a user to effectively switch the active Instance based on a Predicate<IContext> boolean test.  Here’s a quick example of using the new Conditional() syntax of InstanceExpression:

var container = new Container(x =>
x.InstanceOf<Rule>().Is.Conditional(o =>
o.If(c => false).ThenIt.Is.OfConcreteType<ARule>();
o.If(c => true).ThenIt.IsThis(GREEN);


The syntax above is configuring and attaching a ConditionalInstance object.  Internally, this syntax is telling the ConditionalInstance to:

  1. Return the concrete type “ARule” if the condition “c => false” is met (in real usage the predicate would do something more intelligent 😉 )
  2. Else, return a Rule object specified by the variable named GREEN if the condition “c => true” is met
  3. Finally, if none of the predicates match, return the Rule object specified by the variable named RED

The syntax “If( predicate ).*************” uses an InstanceExpression and all possible Instance types are available.

Internally, the ConditionalInstance looks like this:

public class ConditionalInstance<T> : ExpressedInstance<ConditionalInstance<T>>
// Conditional Instance keeps track of zero or more internal Instance
// objects against a Predicate<IContext> condition
private readonly List<InstanceCase> _cases = new List<InstanceCase>();
// The "default" Instance to use if none of the conditional predicates
// are met.  If this is not explicitly defined, the ConditionalInstance
// will simply look for the default Instance of the desired
// PluginType
public Instance _default = new DefaultInstance();
public class InstanceCase
public Predicate<IContext> Predicate { get; set; }
public Instance Instance { get; set; }


When a call is made to container.GetInstance<Rule>(“conditional”):

  1. Internally, the Container object finds the ConditionalInstance object that was configured and named “conditional” for the PluginType “Rule”
  2. The Container invokes the ConditionalInstance.Build(Type, BuildSession) method
  3. The ConditionalInstance evaluates its InstanceCase collection to find the first InstanceCase that matches the current IContext and invokes the internal Instance of that InstanceCase
  4. Lastly, if ConditionalInstance does not find any matching InstanceCase objects, it will invoke its default Instance to build the requested object

It might be easier to just see the code for this:

protected override object build(Type pluginType, BuildSession session)
// Find the first InstanceCase that matches the BuildSession/IContext
var instanceCase = _cases.Find(c => c.Predicate(session));
// Use the Instance from the InstanceCase if it exists,
// otherwise, use the "default"
var instance = instanceCase == null ? _default : instanceCase.Instance;
// delegate to the chosen Instance
return instance.Build(pluginType, session);

Please see Using the Session Context for more information on what is possible with the IContext.

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