View Functions

View functions allow you to query smart contracts on the blockchain. They are defined in smart contracts as entry functions with the view modifier. In this guide, we will provide snippets of view functions and how they are typed and used.

Dynamically Typed View Functions

When you don’t care about the return type of a view function, you can use the View function without any type arguments.

The Move function we will be calling:

public fun balance<CoinType>(owner: address): u64

And to call the view function, we will use the View function from the ContractClient.

using Aptos;

class Program
{
    static void Main(string[] args)
    {
        var client = new AptosClient(Networks.Mainnet);

        // Call the view function by specifying the function name, arguments, and type arguments
        var values = await client.Contract.View(
            new GenerateViewFunctionPayloadData(
                function: "0x1::coin::balance",
                functionArguments: ["0x1"],
                typeArguments: ["0x1::aptos_coin::AptosCoin"]
            )
        );

        // Returns a list of return values: ["100"]
        ulong balance = ulong.Parse(values[0]);
    }
}

Simple Typed View Functions

For view functions with common return types, you can type the return values by passing in a type argument.

The Move function we will be calling:

public fun get_current_epoch_proposal_counts(validator_index: u64): (u64, u64)

And to call the view function, we will use the View function from the ContractClient with the type arguments.

using Aptos;

class Program
{
    static void Main(string[] args)
    {
        var client = new AptosClient(Networks.Mainnet);

        // Call the view function by specifying the function name, arguments, and type arguments
        var values = await client.Contract.View<List<ulong>>(
            new GenerateViewFunctionPayloadData(
                function: "0x1::stake::get_current_epoch_proposal_counts",
                functionArguments: [(ulong)0],
                typeArguments: []
            )
        );

        // Returns a list of return values: ["100", "100"]
        ulong successfulProposals = values[0];
        ulong failedProposals = values[1];
    }
}

Complex Typed View Functions

For view functions with complex return types, you can leverage Newtonson.Json to deserialize the return values. By default, all types passed into the View function leverage JsonConvert.DeserializeObject<T>() from Newtonson.Json to deserialize the return values. You can override the deserialization behavior by creating a custom JsonConverter.

The Move function we will be calling:

public fun supply<CoinType>(): Option<u128>

Create your own JsonConverter to deserialize the return values.

using Aptos;
using Newtonsoft.Json;

[JsonConverter(typeof(CoinSupplyConverter))]
class CoinSupply(ulong value) {
    public ulong Value;
}

class CoinSupplyConverter : JsonConverter<CoinSupply> {
    public override CoinSupply ReadJson(JsonReader reader, Type objectType, CoinSupply existingValue, bool hasExistingValue, JsonSerializer serializer) {
        // The return type of the view function is an Option<u128> -> [{ "vec": [] }] or [{ "vec": ["100"] }]
        JArray array = JArray.Load(reader);
        var option = array[0];

        // If the Option is None
        if (option["vec"].Count == 0) return null;

        // If the Option is Some
        ulong value = ulong.Parse(option["vec"][0]);
        return new CoinSupply(value);
    }
}

And to call the view function, we will use the View function from the ContractClient with the type arguments.

using Aptos;
using Newtonsoft.Json;

class Program
{
    static void Main(string[] args)
    {
        var client = new AptosClient(Networks.Mainnet);

        // Call the view function by specifying the function name, arguments, and type arguments
        CoinSupply coinSupply = await client.Contract.View<CoinSupply>(
            new GenerateViewFunctionPayloadData(
                function: "0x1::coin::supply",
                functionArguments: [],
                typeArguments: ["0x1::aptos_coin::AptosCoin"]
            )
        );

        ulong coinSupply = coinSupply.Value;
    }
}