Inline contract calls

There's essentialy two reasons for interacting with other contracts: 1. to query their state. 2. to create an inline transaction, that is, a new transaction which will be executed after the original transaction.

Both of the two operations can be done in two ways:

  1. using the transaction execution context.

  2. adding a Contract Reference State to the contract, then using CSharpSmartContract.State to call methods.

Using the Context

Query state from other contracts

Let's see how to call the GetCandidates method of the Election Contract and get the return value directly in your contract code with the Context property that is available in every smart contract.

using AElf.Sdk.CSharp;
using AElf.Contracts.Election;
// your contract code needs the candidates
var electionContractAddress =
// call the method
var candidates = Context.Call<PubkeyList>(electionContractAddress, "GetCandidates", new Empty());
// use **candidates** to do other stuff...

There are several things to know before writing such code:

  • Because this code references a type (PubkeyList) originally defined in the Election Contract (types are defined in a proto file, in this case election_contract.proto), you at least need to reference messages defined in the .proto file in your contracts project.

Add these lines to your csproj file:

<ContractMessage Include="..\..\protobuf\election_contract.proto">

The ContractMessage tag means you just want to reference the messages defined in the specified .proto file.

  • The Call method take the three following parameters:

    • address: the address of the contract you're seeking to interact with.

    • methodName: the name of method you want to call.

    • message: the argument for calling that method.

  • Since the Election Contract is a system contract which deployed at the very beginning of AElf blockchain, we can get its address directly from the Context property. If you want to call contracts deployed by users, you may need to obtain the address in another way (like hard code).

To send an inline transaction

Imagine you want to transfer some tokens from the contract you're writing, the necessary step is sending an inline transaction to MultiToken Contract, and the MethodName of this inline transaction needs to be Transfer.

var tokenContractAddress = Context.GetContractAddressByName(SmartContractConstants.TokenContractSystemName);
Context.SendInline(tokenContractAddress, "Transfer", new TransferInput
To = /* The address you wanna transfer to*/,
Symbol = Context.Variables.NativeSymbol,// You will get "ELF" if this contract is deployed in AElf main chain.
Amount = 100_000_00000000,// 100000 ELF tokens.
Memo = "Gift."// Optional

Again, because you have to reference a message defined by multi-token contract proto file, you need to add these lines to the csproj file of your contract project.

<ContractMessage Include="..\..\protobuf\token_contract.proto">

This inline transaction will be executed after the execution of the original transaction. Check other documentation for more details about the inline transactions.

Using Contract Reference State

Using Contract Reference State is more convenient than using Context to do the interaction with another contract. Follow these three steps of preparation:

  1. Add related proto file(s) of the contract you want to call or send inline transactions to and rebuild the contract project. (just like before but we need to change the MSBUILD tag name, we'll see this later.)

  2. Add an internal property of XXXContractReferenceState type to the State class of your contract.

  3. Set the contract address to the Value of property you just added in step 2.

Let's see a demo that implement these steps: check the balance of ELF token of current contract, if the balance is greater than 100 000, request a random number from AEDPoS Contract.

First, reference proto files related to MultiToken Contract and acs6.proto (random number generation).

<ContractReference Include="..\..\protobuf\acs6.proto">
<ContractReference Include="..\..\protobuf\token_contract.proto">

After rebuilding the contract project, we'll see following files appear in the Protobuf/Generated folder:

  • Acs6.c.cs

  • Acs6.g.cs

  • TokenContract.c.cs

  • TokenContract.g.cs

As you may guess, the entities we will use are defined in files above.

Here we will define two Contract Reference States, one for the token contract and one for the random number provider.

using AElf.Contracts.MultiToken;
using Acs6;
// Define these properties in the State file of current contract.
internal TokenContractContainer.TokenContractReferenceState TokenContract { get; set; }
internal RandomNumberProviderContractContainer.RandomNumberProviderContractReferenceState ACS6Contract { get; set }

Life becomes very easy if we have these XXXContractReferenceState instances. Check the implementation.

// Set the Contract Reference States address before using it (again here, we already have the system addresses for the token and ac6 contracts).
if (State.TokenContract.Value == null)
State.TokenContract.Value =
if (State.ACS6Contract.Value == null)
// This means we use the random number generation service provided by `AEDPoS Contract`.
State.ACS6Contract.Value =
// Use `Call` method to query states from multi-token contract.
var balance = State.TokenContract.GetBalance.Call(new GetBalanceInput
Owner = Context.Self,// The address of current contract.
Symbol = "ELF"// Also, you can use Context.Variables.NativeSymbol if this contract will deployed in AElf main chain.
if (balance.Balance > 100_000)
// Use `Send` method to generate an inline transaction.
State.ACS6Contract.RequestRandomNumber.Send(new RequestRandomNumberInput());

As you can see it is convenient to call a method by using the state property like this: State.Contract.method.Call(input).